When it works, speeds are good and browsing is not affected too much. It is not constant, it may be really quick and sometimes quite slow. Overall good.
Unfortunately, after some random time, it becomes impossible to do anything : browsing to new pages, refreshs. When it happens, gpu usage for firefox is high.
Only solution is to restart firefox which is destroying the usability.
It doesn't happen everyday and sometimes it happens multiple times per day...
I haven't tested the Windscribe windows app.

Edit : I finally switched to Opera for the cases when I need privacy. The windscribe extension can perfectly work some days but the others requires way too much restart of Firefox. I leave it with 3 stars because of the monthly 10Gb free limit.