For what it's worth, I made a quick test of a td game and got 1200 enemies (with collisions, pathfinding and turrets with predictive aiming) before the fps started consistently dipping below 60 fps - again, on a computer using tech from about 7 years ago.
sqiddster - airscape hasn't ever had a smooth framerate on my computer (amd Athlon 4400+, vista, 2gb ram, nvidia 9800gt (512mb VRAM)), but I listened to about half a minute of the audio in a level and it didn't seem to be playing abnormally here. No amount of fiddling with the effects or resolution settings helps, so it seems to be because the logic is too demanding (the same performance problems are in the version on the scirra arcade). Btw, you might want to implement the trick I talked about earlier running the game from a shortcut to the exe with --ignore-gpu-blacklist added to it, it wouldn't run with webgl on my computer otherwise.