Thank you for Answer!

I replaced byteCount() by isNull(), and I already profiled my app, But I can't find any bottle-neck.

But I saw some other problems:
My rozessor works with 50%, but if I put other window from other app over my app, than my app works with 20% and it stay on 20% if i do ohter window off.
If I put ohter window over half of my window, than I get better stop time (60 ms)

It is very strange! With ATI it works realy fine, but with NVIDIA come this Problems.

Somebody an idea?

thanks!