It's slow for me too.
But I don't really buy that this is only nvidias fault.
There is an problem with the blur effect on ATI cards too http://osdir.com/ml/kwin/2010-06/msg00016.html
and compiz blur works without big performance hits.
I read it as "its because of wrong
Keep in mind, the blur effect was so bugged, that is was removed until kde 4.5.