my Newgrounds submission has about 200 or so objects on the screen at the same time (mostly static), but still gets about 60 fps on the ipad (though it can lag a bit on mobile phones, but this is probably down to bad programming and wasteful memory management on my behalf). I do get a bit of slowdown when the enemies spin off the screen (due to using rotate, probably), but it's only brief and acceptable.
I have discovered that sometimes just one small mistake in the programming can be enough to half your frame rate. For example, during the early stages of creating my game I accidentally had it set to constantly move one of the sprites to the top of the layer. As soon as I added 'trigger once' to the event, the frame rate doubled.
I did encounter issues when originally trying out your Space N Time game on the ipad (shortly after it was submitted) - I basically got an explosion of noise all at once. When I first added music to my game I got the exact same thing, but soon discovered I had the music constantly trying to play itself again and again. So a lot of the time the performance issues are call by good ol' fashioned inexperience on our behalf. Also, the fact that different browsers seem to have different strengths and weaknesses really doesn't help keep things consistent.
Anyway, best of luck with the competition. It's been really nice to see such a wide variety of games.