Vista Tutorial - DWM - When/how often does the screen date happen?

I understand that DWM composes the screen off-screen before copying to the
visible display. But how often does this happen? Presumably, it must happen
at least every 30ms for video display. Are there any controls over this?

Why am I asking? I am asking because my application software that worked
fine under XP (with layered windows) is not as smooth with higher perfoamnce
hardware under Viata. There are occasional glitches in what should be a
smooth real-time update.

TB