I have added this issue to the tracker:
https://winscp.net/tracker/1957

sent

I'm not sure if my line to your server will be fast enough to show the problem. But let's try this. Can you email me the test account details? You will find my address (if you log in) in my forum profile.

No, I just picked a relatively old one for the logs. I have now tried a few going backwards and 5.17.10 is the last one still working fast, all the 5.18.x ones have the issue. I could provide you with a SFTP account to my server for testing.

Is the 5.13.6 the last "fast" version? Is the 5.13.7 slow? If not, what is the last "fast" version?

sent, both logs with WinSCP, "slow" = latest version, cancelled the upload after a few seconds.

You can attach them here privately.

Same server, same file:
5.13.6: 39 MB/s
5.18.2: 730 KB/s (400 while logging)

Where can I send you the WinSCP log file? (don't wanna attach it here publicly)

Please post verbose log files both from WinSCP and FileZilla.

If you download the older version of WinSCP, is it as fast as it used to be?
https://sourceforge.net/projects/winscp/files/WinSCP/

Nope, logging is disabled.

Haven't you enabled debug logging?
https://winscp.net/eng/docs/faq_slow#logging

Hi, been using this great app for years but lately, not sure since which version but I think the previous 2-3 updates, SFTP has become really slow. Uploading a file to my server gets around 680 KB/s, whereas the same file/connection with Filezilla gets 40 MB/s.

Is there anything I can do to help troubleshoot this issue?

Currently using 5.18.1 beta 11089