Comment 3 for bug 1010898

When I switch to the nouveau driver I have the same behaviour as you with Intel graphics.

But do we know where transparency is calculated, do you know?

Assuming it uses the CPU, then the reason why nouveau and intel drivers don't utilize the CPU is as you said a lower frame rate such that transparency calculations don't need to be done that often. But then there must be some bottleneck elsewhere. The drivers readback? (If it's done on the CPU it must use the readback, because the already composited stuff isn't found anywhere in the CPU memory - right?).