I quickly ported JavaGear to Actionscript 3 the other week to see how fast it would run. The actual port was really quick - took just over a day to get it up and running.
It seems that Actionscript is a pretty sluggish language for emulation purposes. The main bottleneck in the code is the Z80 emulation. The rendering and flash specific stuff is not really an issue.
Using a hacked version of Sonic, with some patched speed enhancements (replacing some of the idle cpu loops with halt instructions), it's just about playable. But I'd be lying if I said I wasn't disappointed with the overall performance.
I did a lot of profiling, but so far haven't made any breakthroughs. The core code is pretty fast anyway. I tried an alternate Z80 emulator from a Flash Spectrum emulator, and this was slower than my own.
Edit: give it a spin here
Be warned it's slow, so save any important work in case it stalls your PC ;)