WinSCP slow speed SCP/SFTP – How I fixed it

Hi folks,

old servers I knew were capped at 10Mb/s, so when WinSCP reported 1MB/s I was actually happy with it, so I stopped paying attention to it.

Recently I noticed that even from 1Gb/s and 100Mb/s servers, WinSCP was still downloading at 1MB/s so I got curious and then little frustrated when I noticed culprits were always the same servers, while others ranged from +4.5MB/s.

I changed most relevant settings but the one that actually worked was to disable Connection -> Optimize connection buffer size.

Cheers

Thanks for sharing your solution.

I wanted to also thank the OP for the solution, and to note that my transfer speed went from ~550 KB/s to ~2.8 MB/s with no other changes but this single tweak. Should this be noted on the FAQ entry for this? I found it via FAQ -> Bug 164 -> Here, but I feel like not everyone would follow through that trail :)

marcolussetti wrote:

Should this be noted on the FAQ entry for this?

Are you referring to some specific FAQ entry? Or are you asking for a new FAQ entry?

Not working for me.
Versione 5.11
I noticed that SCP is much faster when transferring files.

Worked for me here also. It is now several times faster than it was before I changed the setting.

wow... this worked for me also. I was copy files on my LAN with scp and it topped out at 1200kb/s, and after I did this, I am now copying at 32MB/s.

MarcoP wrote:

I changed most relevant settings but the one that actually worked was to disable Connection -> Optimize connection buffer size.

Thank You!

This needs to be in the FAQ & perhaps a dev should take a look. Went from 300kb/s to 10,9MB/s (connection cap). This fixed my problem. HERO

EDIT:
This FAQ topic
https://winscp.net/eng/docs/faq_slow

OK, I've mentioned the option in the FAQ.

Spend like 300GBP for new switch because was thinking maybe my network issue tweak all setting on my gigabyte card update firmware.

Speed test show full gigabyte speed, but winscp (newest) was slow max 8-13MB/s 10 diffrent servers centos ubuntu xserver my datacenter backup dirffrent configuration scp sfpt ...

Then install FileZilla spft =====>>>> and get 32MB/s

This is official only one solution

You waste my time never in my life i will be use winscp !!!!!!!!!!!!!!!!!!

Hahaha your lack of debugging disturbs me. As does your willingness to buy random hardware to solve your issue. This is a opensource project as is filezilla you should thank the developers not berate them when you have an issue.

It works after I turned it off. It was about 800kb/s and now 32M/s. Thank you for your solution!

Is this a GUI only solution? I have a third party software workflow that utilizes WinSCP scripting under the hood. Would there be an equivalent setting in some config file somewhere? Thank you!

savvykong wrote:

Is this a GUI only solution? I have a third party software workflow that utilizes WinSCP scripting under the hood. Would there be an equivalent setting in some config file somewhere? Thank you!

Equivalent to what?

martin wrote:

Equivalent to what?

A scripting equivalent to the "Optimize connection buffer size" checkbox in the GUI. But after reading through WinSCP documentation further I found what I was looking for - "SendBuf".

i always noticed this with winscp, when i compare downloading something with filezilla or the old flashFXP client, winSCP always seemed slower as if it was capped, so im glad i found this subject here, but when i go into my site manager/advanced, i can see the optimize option but its greyed out and i cant untick it.
i have a passive option with can be ticked/unticked

glenhall wrote:

i always noticed this with winscp, when i compare downloading something with filezilla or the old flashFXP client, winSCP always seemed slower as if it was capped, so im glad i found this subject here, but when i go into my site manager/advanced, i can see the optimize option but its greyed out and i cant untick it.
i have a passive option with can be ticked/unticked

The option is not available/relevant to FTP protocol.

MarcoP wrote:

Disable Connection -> Optimize connection buffer size.

Awesome. This brought my local Ethernet performance from 750KB/s to 35MB/s(!!!)

Was getting ~ 300-400KB/s, to servers I knew could do better. Unchecked that box and immediately got 11MB/s to the same host.

I have the same problem but can't fix it. Why?

crazyluke wrote:

I have the same problem but can't fix it. Why?

See https://winscp.net/eng/docs/ui_login_connection

Disable connection optimization dramatically improved performance for me. Going from 400kbps to 4MB/s

crazyluke wrote:

I have the same problem but can't fix it. Why?

I had some issues finding it as well. On the specific connection click the Advanced button and in the Advanced Site Settings window select Connection.
Under Connection uncheck Optimize connection buffer size.

How do you access these settings? When I open my settings panel it doesn't have this sub-cat???

It's here:
https://winscp.net/eng/docs/ui_login_connection

Thanks, went from around 300 KB/s in a VM to 30MB/s using this... that's a 100X upgrade...

I discovered for me, I was using a Proxy config to tunnel through a bastion host with plink. I converted that to use the built in Tunnel config and that sped up throughput immensely.

Did nothing for me.
I still download with 20kb/s on a 150 Gbit connection.

I've struggled with slow speeds from my ESXi servers for years. I just thought that there was something going on with those servers that caused the slow speeds. I just found this thread today, and my transfer rates went from 400 KB/s to 25 MB/s.
I appreciate the great product that the developers put out, but it sure seems like maybe this setting should be off by default (since it impacts so many different people).

JimBurd wrote:

I appreciate the great product that the developers put out, but it sure seems like maybe this setting should be off by default (since it impacts so many different people).

Actually, we believe that the default makes it faster for most users.

I was getting about 10 KB/s with it enabled. Disabling it gets me to about 50 MB/s.
One difference I noted is that the CPU utilization on the remote host I was transferring from was pegged at 100% with the option enabled. After disabling it I see about 50-60% CPU utilization.

where does one find "Connection uncheck Optimize connection buffer size" setting for latest version? there's no connection settings listed on left like in the image?

gsds2006@... wrote:

where does one find "Connection uncheck Optimize connection buffer size" setting for latest version? there's no connection settings listed on left like in the image?

I've posted a link at least twice above already.
https://winscp.net/eng/docs/ui_login_connection

martin wrote:

I've posted a link at least twice above already.
https://winscp.net/eng/docs/ui_login_connection

The issue when I do it is that option is greyed out, how do I get passed that part?

the corey's wrote:

The issue when I do it is that option is greyed out, how do I get passed that part?

The option is mostly relevant for SFTP and SCP protocols only. What protocol are you using?

Hello,

I have a relatively fast freenas server connected via 10G with my PC.

When transfering a big systembackup file towards the NAS I noticed that the transfer rate was only about 640 gbit. I did at expect something like three to four times that.

So I looked at the cpu load of the nas (moderate intel Pentium g5460, 16GB) which was about 14% ….. and did have a look at the much more powerfull cpu here on my pc (AMD2700X 32GB). And up to my surprise my PC seems to be the limmiting factor ……

The taskmanager showed me that one of the possible 16 threads was at 100% all the time. No doubt that that was related to winscp …..

So there seems to be "some" room for improvement:
- starting with migratio to 64 bit
- and using multiple threads

I understand that you are using standard librarys …… but never the less I hope that you can improve things. By the way I did turn of the buffersize optimalisation, that was a slight improvement

Sincerely,

Louis

louis2 wrote:

When transfering a big systembackup file towards the NAS I noticed that the transfer rate was only about 640 gbit. I did at expect something like three to four times that.

So I looked at the cpu load of the nas (moderate intel Pentium g5460, 16GB) which was about 14% ….. and did have a look at the much more powerfull cpu here on my pc (AMD2700X 32GB). And up to my surprise my PC seems to be the limmiting factor ……

Thanks for your report.
What protocol are you using? What version of WinSCP?

Hello,

I am using SFTP and the very latest WinSCP version
Sincerely,

Louis

Louis2 wrote:

I am using SFTP and the very latest WinSCP version
Sincerely,

Thanks. Can you start a new thread and post a log file?

Holy cow!!! Thanks for sharing this!
I tried so may things to speed up this download, but it was taking forever. At hour 4 I decided to dig harder and ran across your post here. After unchecking "Optimize connection buffer size", and restarting my download... 5 MINUTES! I burned 4 hours when it should have taken 5 minutes. I can't thank you enough for this tip.
-Dana

My WinSCP on a Raspberry Pi3 got from kb to MB with:
/boot/cmdline:
/boot/config.txt:
Also look at: <invalid hyperlink removed by admin>

The big differents was at the cmdline.
Test first with:

Hi!

This fix also helped me a lot! I was downloading at ~500KB/s, now I'm downloading at full speed: 44MB/s.

I would recommend disabling this option by default and showing an alert whenever the user is trying to connect with this option enabled (and a checkbox to disable that alert in the future).

I'll repeat: from ~500kbps to 45,3MB/s (It grew while I was writing this!)

panreyes wrote:

I would recommend disabling this option by default and showing an alert whenever the user is trying to connect with this option enabled (and a checkbox to disable that alert in the future).

I'm pretty sure the option speeds up the transfers for most (that's why it is on by default). It's probably only on some rare network setups, or possibly buggy network drivers, where disabling the option helps.

I know this is an old thread, but the above changes did not help me in my case. What I found was the ordering of the ciphers played an important part in speed for SFTP from the vendor we were connecting to. At most I could get was 5.5Mb~ but after the below changes we were up near the 25.5Mb~ range. I would suggest adding a mention to ordering the supported ciphers to the FAQ.
Raw settings for my session options were changed from
to
Came across this article that helped explain the impact of the ciphers used on SFTP speeds which helped us to improve the transfer speed.
https://www.systutorials.com/improving-sshscp-performance-by-choosing-ciphers/

Fresh install of Windows 10 + Latest winSCP today (v5.17.7 Build 10640) and I see this problem still an issue years later, in 2020!

Over the last few years I've used WinSCP on multiple different computers and different networks and disabling this option ALWAYS improves my SCP transfer speeds by more than a factor of 10. Today I went from 2KB/s to 60MB/s.

Add my name to the list of people who really think this option needs to be disabled by default. In all of my installs, I've yet to see a single instance where this option improved speeds.

Thank you for your work on WinSCP. Other than this annoyance, it has been a great tool that I've used for years.

linuxdave wrote:

Over the last few years I've used WinSCP on multiple different computers and different networks and disabling this option ALWAYS improves my SCP transfer speeds by more than a factor of 10. Today I went from 2KB/s to 60MB/s.

Are you really always using the SCP protocol only?

martin wrote:

Are you really always using the SCP protocol only?

Yes. 100% of my usage is SCP protocol only.

linuxdave wrote:

Yes. 100% of my usage is SCP protocol only.

Can you test it with SFTP?

Hi guys

I have tried both SFTP and SCP with connection buffer size optimization disabled, and it doesn't improve my slow (about 30x slower than the network) transfer speeds. Are there other common causes for slow transfer speeds that I should consider?

Thanks!

If you are using scheduled task try to change priority
https://aavtech.site/blog/windows-task-scheduler-changing-task-priority/

It works for me too (I have a VPS). The speed increase is considerable (+ 200%)

@MarcoP: you saved my life !
… well, my transfers really . . . !

Wow, I have now a speed of 68Mbyte/s instead of 800Kbyte/s.

You are a life saver!

Cheers!

From 5–7 MB/s to 20–30.

Taking into account that I usually transfer files in units of multiple GB I don't know how I didn't search for this earlier.

Thank you very much.

This fix of disabling Optimize buffer size has increased speed from 6 mb/s to 20 mb/s. Very grateful

I disconnect from ExpressVPN and the transfer rate increased 10 times. I tried again with the VPN connected and again without the VPN. This confirmed my first finding.

BTW, I couldn't open video streams while using WinSCP to upload via ExpressVPN.
It's time I finally start looking for a different VPN.

working on 2022

Like many others, this worked wonders for me. Transferring from an Ubuntu Server on my local network to my Windows 11 freshly installed machine across a 1Gbps network. Prior to this change, it was starting at about 12MB/s but with every passing second it was decreasing, down to the range of a few KB/s and was going to take hours to transfer the 540MB file.

Once I turned this optimise connection setting off, it transferred at 100MB/s and naturally, was done a few seconds later.

I have read the link (ui_login_connection) and found only a small difference.

WinSCP: v5.21.7 Build 12963
OS Client: MS Win11 Enterprise v22H2 Build 22621.1265.
OS Installs: True MS-Image and Corporate Image.
Hardware: Various
Network: Flat 192.168.0.0/24
Protocols Used: TCP, UDP, SMB, NFS, and all associated others.
FTP Server: Qnap NAS v4.2.6 latest and last Build. Unit is EOL. I also tried other FTP servers on other operating systems.
Experience: 30yrs+. Desktops to OC48 Fiber, Data Center design and build. Engineer and implement all aspects of "IT"

The issue started with building a data connection via MS FTP cli. I could connect and authenticate with multiple test accounts with varying rights. All are unable to (can transverse directories, i.e. cd\ directories.) but when issuing a "Dir" or "ls" and waiting, I would receive an "Ftp error 425: Can't Open Data Connection" I have tried all Tech bulletin's and articles without success. Other OS's work without issue.

So after maintaining my flat spot of banging my head against the wall I figured it's time to load a GUI app. So here I am. With WinSCP I am able to transfer data, both compressed and uncompressed but at very low speeds of 25kBs +/-. Any help would be Greatly Appreciated.

I really thought there are some issues in my network., This fixed transfer rate from kilobytes to ~20 MB/s. Thanks OP!

old thread I know, but...
am getting 10kbps when using SFTP from remote CentOS7 box to local Win10 client.
found the optimise setting from the error prompt prior to this page, sadly no change when set as reccomended above.
other protocols fly, host has 10Gb, I'm on 1Gb.
wondered if anyone would suggest where else to look?
within WinSCP..?

('kin awesome program btw, many, many thanks)

regards,
scott

I am facing very slow speeds trying to download some files. The speed cannot exceed 5 kb/second. I then get an error that the host is not communicating and it is disconnected. In fact, for 300Mb it says it is going to take 18 hours. I tried to turn off the OPtimization connection buffer size. Change from SFTP to SCP. Choose a different encryption file but nothing worked out. I am not very familiar with all of these but I cannot detect what might be responsible for that and it is kind of urgent to download my files.

@dkary: Please start a new thread and post a session log file there.

While not solving everything it did take me from 0.95 MB/s to ~25 MB/s so a huuuge thanks OP

Martin, I wonder if a good idea would be to have a button to speed test? Automated test various settings and file upload / download to see what works best.

This looks like enough of a problem for people that it may be worth doing. As far as I know, WinSCP doesn't have this.

Just an idea. People shouldn't have to figure this stuff out =)

@Guest: Thanks for the idea, I'll consider it.

Version 6.3.6 (Build 15073 2014-11-25)

So I have been using WinSCP (recently paid!) for years and I finally started to wonder why I could download multiple queued files at identical speeds simultaneously.

I turned off "Optimized" in the Connection settings and the downloads, while they were eventually faster, the initial "run up" to the maximum speed was longer.

I read through the FAQ and followed the "raw settings" adjustments. A point to note is that the "Common" settings do not include the SFTPDownloadQueue or SFTPUploadQueue. It would be preferred if they were included in the list of explained options here: https://winscp.net/eng/docs/rawsettings

I searched more and found this post:
https://winscp.net/forum/viewtopic.php?t=4366#14109

In which the SFTPDownloadQueue setting is advised to be changed from 4 to 10, so I tried that and the performance was reduced from 3MB to 1.1MB. So that was not a good outcome.
So then I tried changing SFTPDownloadQueue to 50 and was able to achieve as high as 5.8MB/s

The results of my SFTP download tests between WinSCP, the sftp command on both WSL and Windows CLI natively, and FileZilla were as follows using the same source and target machines:

Optimized BUFFER Size turned On.

• WinSCP (Optimization On): 3.1MB
  
• WinSCP (Optimization Off): 3.5MB/s
  
• WinSCP (SFTPDownloadQueue=50): 5.8MB/s
  
• WinSCP (SFTPDownloadQueue=100): 9.2MB/s
  
• WinSCP (SFTPDownloadQueue=1000): 11.3MB/s
  
• WinSCP (SFTPDownloadQueue=1500): 12.6MB/s
  
• WinSCP (SFTPDownloadQueue=2000): 11.8MB/s (slow (2-3s) to cancel)
  
• WinSCP (SFTPDownloadQueue=5000): 1.6MB/s (slower (10s) to cancel)
  
• WinSCP (SFTPDownloadQueue=200, Upload=150): 11.8MB/s
  
• WinSCP (SFTPDownloadQueue=500, Upload=500): 11.2MB/s
  
• WinSCP (SFTPDownloadQueue=1750, Upload=1000): 10.5MB/s (slow (2s) to close)
  
• SFTP: 6MB/s
  
• FileZilla: 11.5MiB/s

Speedtests on my connect register over 100MB/s

If Optimized Buffer is turned off with the raw settings still enabled the speed takes a while (10–20 seconds) to reach "top speed" and the performance is about the same as with optimized turned off. Frequently the speed will drop lower than the average throughput reported with optimized enabled, but top speed will reach around the same as with optimized turned on.

Is SFTPDownloadQueue the optimal way to improve the speed, and if it is, why isn't it optimized?

I can queue up and start multiple simultaneous downloads of files in WinSCP from the same host and they all download at at the same capped out speed as the individual single file transfer. Transfer Limit is set to Unlimited.

@RR: Note that the current default for the queues is 32 (not 4) for quite a while:
Issue 1295 – Optimize SFTP transfers on high latency connections
So 10 is indeed a decrease.

Seems like it should be close to 1000 to provide adequate throughput for high-speed connections, not 32.

I've set
and found the performance to be nearly 10x better than the default setting.

I found the default whether that was 10 or 32 to be quite inadequate. The ability to change the queue buffer should be more obvious for new user because.

Personally, if I would not have found a solution to this issue by editing the "raw settings" I would have migrated to using another product because I want to receive the highest throughput possible.

FileZilla is open-source, is there not a way to understand how their out-of-the-box performance is nearly 3x WinSCP?

@RR: Is FileZilla really 3x faster for a single file connection for you?
Can you post some logs?
Anyway, FileZilla has quite different architecture/design. They focus primarily on performance. While we focus primarily on rich features. It's hard to have both. It's your choice, what you prefer.