When deploying speedbrakes with default FSX planes you can see the speed drops very fast, the plane goes nose-up and looses stability like it's stalling. You have to use them like 3 or 4 seconds to already get your desired speed. It's not realist at all.

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.