(In reply to comment #9)
> No effect I'm afraid. Since this is a full screen application I assume it
> was already using flipping and not blitting anyway?

Probably. For the sake of testing, have you tried disabling page flipping in
addition to SwapBuffersWait?

(In reply to comment #12)
> Down to dri2_drawable_get_buffers() now. I assume I'll be hitting a point
> where I'll have to switch over to looking in the X server soon...

Yep, it's waiting for DRI2 buffer information from the X server, which is
delayed until the previous buffer swap actually finishes.

FWIW, you might get somewhat less confusing timings if you call glFinish()
before glXSwapBuffers().