Thanks for your report.
I have sent you an email with a debug version of WinSCP to the address you have used to register on this forum.
Going back to version 5.13.8 solved the issue, it's running smoothly again. So there's indeed something wrong with the newer versions.
I just updated to version 5.17.9 on Windows 10. Things were running really smoothly before, but not WinSCP is much slower, it often freezes for several seconds at a time. I have a PhD in computer science but I don't have more time to provide a more detailed report. I hope this is useful feedback. This is such a pain that I'm looking to downgrade to the older version.