@kewe65: Thanks for sharing your findings. What version of Windows is that?

I have WinSCP on two machines, both of them workstation class PC and laptop, but the PC download speed would never go over 400kb. The settings in the app were identical, including the optimization checkbox, which really had no affect on the laptop download speed. The PC has a 10gbe adapter wired to fiber internet.
The PC had a brand new Win11 installation.
What I discovered after a lot of internet searching is this setting called "Receive Window Autotuning level". In cmd prompt, enter netsh interface tcp show global


Changing that from disabled to normal fixed the download speed in WinSCP, not to mention overall as the up/down is back to where it was with the 10gb fiber service initially at 4.5GB

The source article: https://fossbytes.com/speed-internet-using-command-prompt-cmd/

Just realized how perverted this setting was so that I had to undo the "optimization" to get the speed? It has indeed boosted the transfer rate 10 fold (and some more!)... You know guys, if you have an adaptive "optimization" algorithm that allows the transfer rate to drop while the transfer is in progress, such an algorithm should be trashed. It's clear as a day that it reports faster speeds at the beginning and then it is doing the "buffer optimization" that drops the speed at least 5 times (for me)... And what kind of "optimization" is that, actually LOL? Sensing the speed drop, it should at least stop "optimizing". It's so backwards, really!

And oh, "Advanced..." has the "Optimize connection buffer size" box checked.
I strongly believe that that's a WinSCP issue because NOTHING else network-bound is as slow on this machine as WinSCP... Sad

Did you read what I said? Command line scp from the same file source gives at least 10x faster speed on the same machine, same everything (encryption, CPU intensity, blah blah blah -- it's all the same). And no, debugging is not enabled in WinSCP. I noticed that WinSCP starts at showing 2.9MB/s then in a few seconds it settles down at around 600KB/s, which is just ridiculous for speed.

Did you check the hints here?
File transfer speed is very low. WinSCP does not utilize all available bandwidth. How do I improve the transfer speed?

I just tried to download a file from remote and WinSCP was showing download rate 500kb/s, I cancelled and restarted a few times, it was all the same (ETA was shown like 30 minutes). Then I just did scp from a cygwin command line prompt and it was going at almost 8MB/s, which is what I was expecting, and the file was here in 3:02 – that's three minutes). That's unacceptable for WinSCP to have such a poor performance, no matter how fancy the UI looks like.