Hallelujah! The command "/set maxfpsbk 0" solved the problem, the CPU usage dropped to the good ol' 5 %!

Yes, now I can waste all my spare time in front of a silly computer game again.

Thanks for the advice Bena!

So what do you think about my new paladin tank, arms warrior, retardin, fire mage, resto shaman group? Will it rule the world?