Ubuntu does not enable metacity compositing by default so if things were at defaults metacity was not using compositing. Also, if things were kept default, compiz was not using the Unredirect Fullscreen Windows option (like the kwin option people keep talking about) so this was with full compositing overhead. We disabled that option because it causes too many other issues with screensavers activating while playing games and causing weird things to happen to non-game fullscreen applications.
Nvidia blob does not suffer much from the extra blits required, as these are very optimised. Don't extrapolate from your system to everyobody.
I extrapolated nothing. You took from my comment what you wanted to. Read my response again (this time more thoroughly) and you'll see that I pointed out that it was possibly due to the hardware I was running that I have no problems with Compiz.