Comment 23 for bug 1037411

(In reply to comment #3)
> Yes, the read buffer is GL_FRONT in this case. So I guess the slow-down is by
> design in Mesa.

Rather the X server / DRI2 protocol. It *might* be possible to make xserver not enforce the fake front buffer for the Composite Overlay Window, not sure.

> I'm going to work around it in compiz anyway. glCopyPixels
> should never be touched at all really.

Out of curiosity, what are you using it for?