Not trying to hate on KDE, I know many people who love it, but I have found it less performant on old hardware than Xfce. (Maybe its a cpu/GPU rather than ram thing?) I started using Linux last year, and tried KDE last month.
If the hardware is really old to not br able to support animations properly, just disable those animations. Transparency effects can be disabled as well. Everything. It is by far the lightest and most efficient DE
