I think the speed will vary from each implementation of webkit, the best would be to use c++, the python/GTK version plays Space Blaster with no lag but I couldn't get sound working.
I used Sunspider to do some bench marking as it also does 3d tests.
Comparison Webkit C#(OpenWebkitSharp) Chrome(13)
3d: 2.45x as fast 102.8ms +/- 12.4% 42.0ms +/- 8.1%
cube: 2.54x as fast 32.8ms +/- 34.4% 12.9ms +/- 22.8%
morph: 2.49x as fast 33.3ms +/- 40.3% 13.4ms +/- 8.4%
raytrace: 2.34x as fast 36.7ms +/- 39.2% 15.7ms +/- 7.4%
As you can see Chrome is consistently twice as fast.
But as far as 3d acceleration goes, I'm doing this on a laptop running XP with a 3D card but not sure if it's being used. But for what I have read webkit does do 3D acceleration.