then an additional ~8 ms is spent blocking on buffer swap (= waiting for the driver to complete the queued commands, e.g. C code (or something) in the driver)

As a rule of thumb, if you're blocking on swap/flip/present, you're either waiting for a vblank if you're vsyncing, or you're waiting for the GPU to catch up.

Measurements of "GPU load" are very misleading. You can be bottlenecked by the GPU without seeing it report 100% load...

Threaded optimization off:

51 FPS

Threaded optimization on:

61 FPS

GPU load: ~71%

I did not change a single line in my program. Threaded optimization on simply moves the cost of the render calls to the driver server thread (see the slides I posted above), and if the server thread lags behind it causes it to block on buffer swaps.

Remember to use timestamp queries for GPU timing. tmason, I think what you want to do is put all of your material parameters together in an array, and load it into a single buffer. Then you want to make a single MultiDraw call, and use DrawID in the shader to choose the correct material. That way you won't have that loop of calls anymore.