It controls for how much data will WinSCP ask the server for before waiting for a response. With high latency connections (in your case the slow network drive has the same effect as a high latency), higher value is needed to saturate the bandwidth.

Thanks Martin! I'll play around with different values and see how it goes. What does SFTPDownloadQueue do exactly? What's the default value? What's considered a really high value?

Thanks for your feedback. You can try even larger value.

Thanks Martin! That seems to have helped. Transfer speed increased from 5 MB/s to 26 MB/s when downloading a file over SFTP to the Samba share. Seems like something still isn't quite right though, because downloading to SSD is still much faster, consistently getting 90MB/s with that test now after many test runs.

Sorry for my late answer. Can you try setting SFTPDownloadQueue=64 in raw site settings?
https://winscp.net/eng/docs/ui_login_raw

Attached the log repeating the same steps but with Debug 1 logging. Thanks again!

Thanks. Can you please increase logging level to Debug 1?
https://winscp.net/eng/docs/ui_pref_logging

Attached a session log of first downloading 1GB file to Desktop folder (SSD) at 30 MB/s and then downloading to Z:\ (network drive) at about 5 MB/s. Thanks for taking a look!

Please post session log files both for the download to SSD and the network drive.

Client side is Windows 10 running WinSCP 5.17.10. Server side is Debian 10. I've got a network drive mapped in Windows and noticed that when downloading to the network drive (Samba), speed tops out at 5 MB/s. However, if I download to SSD instead, I'm getting 50 MB/s.

As a quick sanity check I tried Windows file copy from the SSD to the network drive and it saturates the 1 Gbps link @ 107 MB/s.

I tried toggling "Optimize connection buffer size", and "Enable compression" but didn't see any improvement.

I tried the same test with FileZilla and got 50MB/s whether it's downloading to the SSD or to the network drive. However, I'd prefer to use WinSCP because of the nice scripting capabilities.

Is there any way I can improve the transfer speed when saving to a networked drive? Thanks!