I have a pretty decent core i7 740 machine for development. It is usually hanging around 2 or 3% cpu usage in idle. But after I start Intellij - even with an almost empty project (just a couple of hand written classes), compiz shoots up to 20% - and stays there indefinitely.
I noticed this problem because i would be doing something else and notice that the fan would come on - even though my laptop was not really doing much. Then on "top" I see compiz 20%. After shutting down other programs it was isolated to IJ (using 11 ultimate). This is repeatable: i have shut down machine, then watched "top" (2-3%) then started IJ (and waited for it to stabilize). Minutes later it's still at 20% - even though there is no work being done (e.g. indedxing). Again: this is an almost empty project so almost zero indexing was required anyways)