R. Aranda S. wrote on Jan 27^th, 2012 at 7:00pm:

CS 767 is realist on that point. Like in real aircraft, speedbrakes take a little time to slow down the plane, specially if you go heavy.

Since I have not piloted a real 737 or 767, I'll take your word for this. I do know from a passenger point of view that the effect of deploying the speed brake is felt significantly and makes quite a bit of noise. As I mentioned, the effect on the CS767-300 is negligible. or I am doing something wrong.