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/