WinSCP File transfers limited to ~20Mbytes/s per file.

Hi,

There seems to be a limit set somewhere I'm trying to transfer files via WinSCP v6.3.3.
I can do multiple 20Mbytes transfers simultaneously, it's set to 4 it goes around 80+ Mbytes/s. But if I transfer just one file its still stuck at 20 per file for some reason, I tried disabling optimize buffer size but it makes no difference.

I transfer from a Raspberry Pi 5 device to a Windows 10 machine.

File transfer protocol: SFTP-3
Cryptographic protocol = SSH-2
SSH implementation = OpenSSH_9.2p1 Debian-2+deb12u2
Encryption algorithm = AES-256 SDCTR (AES-NI accelerated)
Compression = zlib

Compression is currently on but it makes no difference with it off either.

The use multiple connections per file I'm not sure works really here, if it did it should accelerate the transfer.

Is one of the CPU cores maxed out during the transfer?
Multiple connections per file are supported for SFTP and downloads only atm. Is that what you are doing?

I've done a little more experimentation. With everything off (compression and buffer optimization) i seem to be getting the best per thread performance ~21Mbytes/s

I can't see a single core being maxxed out as far as i could check.

However i did find the multithreading for single files only occurs when downloading a single file only and it splits it into parts so the threading is directly on the disk the recomposes the file after.
My expectation was that this was multiple connections in a single file download buffered to memory and written to disk seamlessly, but i guess this is a SFTP protocol limitation.