On my desktop I’m with fully upgraded 12.1 RC1(should be RC2 now), using the nouveau driver(desktop effects on).
Playing with colorizing oxygen icons, which mean a lot of “save” and “save as” dialogs from Gimp, KWin become very slow(open KickOff menu took nearly 10 seconds). X itself consumed around 700Mb 
Did some of you notice similar behaviour with GTK programs or nouveau?
P.S. Don’t have this problem with Firefox and Crhomium so it may be just Gimp, but then I do not open lots of dialog windows whith web browsers.