Its amazing how disturbing a performance glitch in your prefered desktop can jar your nerves. One of my Gnome 3 desktops - well the only working one now - since bloody Gnome 3 and Ubuntu have done a disappearing act so that all I get is my desktop wall paper when I log in - I digress thats another story. Anyway I recently noticed that once I have a window open in KDE Plasma - the desktop cube stutters when I move to another desktop, when there is nothing on the desktop its smooth as butter - hardly useful though. I have Linux Mint Katya running on another computer and the smoothness of Compix is a wonder to behold - 6 desktops with Eclipse, browsers and all sorts and the thing just races. This anoying behaviour fo KWin irritated me to the point where I started to work almost exclusively on Linux Mint Katya. Then I remembered that I had once experienced this when I was customising another computer. The cube was smooth until I chaged the Window decoration and then the styttering between cube faces started. When I changed it back all was well.
Well it seems thas was the same issue - I changed my WIndow decoration to deKorator and that was that - My KWin is smooth as butter again. You might want to try that if your desktop animation is not as smooth as you would like it to be