Video card: ATI RV710 (Radeon HD 4350) Enlightenment: E18 83478 (but e17 release is same), composite engine - OpenGL.
When xserver-xorg-video-radeon driver is used, e17 uses ~1-3% CPU. It is OK.
With fglrx driver (AMD Catalyst 13.1, but same status with previous versions) CPU usage by Enlightenment is 3-90% (average 20-30%).
I open the issue, because fglrx driver is faster than provided by xorg. Compiz with fglrx works fast and do not take CPU.