I use Fedora KDE Plasma and it's wonderful, but my machine is pretty new compared to what you've got there. As for widgets and panels, what do you mean complex? I've had no issues with them.
I have tons of issues on Plasma 6 with the whole thing, several bugs reported. Compared to something like cosmic panel widgets, or GNOME or anything else it just seems overcomplex and fragile. I can also imagine this results in lag and performance issues, but idk.