I'm actually running KUbuntu 20.04 on one of my machines right now and it's honestly really good (outside some occasional annoying shit that'll be ironed out eventually I hope), but I'm running it from an SSD with fairly decent hardware so it's hard to say anything about performance.
This is not a slight against KDE, but without a tiling WM I'm maybe not enjoying it enough to switch from Windows.
It's one thing that I can't do my work on Windows (without a VM or containers), but my workflow being completely dead in the water with KDE or xfce is something else.
This is not only about fragmentation, it's also about philosophy of WMs/DEs.