In speed tests I usually measure how many objects I can get before I hit V-sync rate (for me, 75 fps). I can get about 1000 here. You should be careful when using events in speed tests, because rendering simple sprites is trivial for the GPU, so you might end up CPU capping your test with the for each loop.
I'm on an 8800GT with the in-development version of 0.99, which has a faster renderer. I get no sudden drops, it steadily falls from 75fps at 1000 to about 35fps at 2000 which is what you'd expect.