Vitus's normal computer is a 2011 Macbook Air i5 w/4g of Memory.
Installed new Mountain Lion OS X yesterday. Running latest Chrome browser (21.0.1180.57 beta) with around 37 tabs open, some with flash, but letting the activity settle down, we get ~94% idle cpu with activity monitor using ~4% of it.
Somebody along the line massively fixed something; there are other rumors of ML offering performance enhancements.
Good job Apple! ('bout time...).