xfwm4 compositor really slow

[jaanus]

At last I got xfwm4 compiled with compositor support but the result was 
not what I was hoping for. Comparing to xcompmgr, it is slower in times. 
Running glxgears with xfwm4's compositor and xcompmgr, the results were 
like day and night. I do have an old nvidia geforce2, but it shouldn't 
be *that* slow? Tried removing shadows, but no luck. Tried removing 
panel transparency, but I failed (hidden config changed?). Any clues why 
it is so slow here?