I've tried XFCE and thunar(both of which are incredibly fast and I loved them), but I've switched to KDE this year because there was always one annoying bug (last one was with large screenshots https://news.ycombinator.com/item?id=20674473 ). I've also tried LXDE but the window manager seemed very basic compared to XFCE, but I haven't tried LXQT.

I'll do another set of tests once Ubuntu (or rather PopOS) 20.04 LTS releases.