In AS3, Flash calculations are actually reasonably fast. Bad performance is almost always due to rendering - everything is anti-aliased and vector-rendered without GPU support. This appears to be changing with the introduction of GPU acceleration for Flash video.
I would say that the ideal development platform would depend on the kind of game you want to make. Flash is a bad choice for a SNES-style sprite based platform game - maybe python and some libraries would be a good idea here? For a 3D game I would definitely suggest unity3D.
There is the obvious advantage that Flash can be embedded in web pages so the TASes can be watched online, without a hefty video download. Same situation with unity3D, provided the user has a small plugin.
I have seen a lot of community-led games die though. They need an able, enthusiastic leader.
edit: I also have some ideas about what features should be in a game designed for TASing!
super synch-robustness - it would be nice to be able to stitch large portions of runs together, and hopefully not have to deal with lag-frames, loading times or luck-based events (in fact, a new random function would have to be specially written to ensure determinism)
lots of little tricks - TASes are great because of abuse of all the little features the designers put in.
open world/non-linear - route planning and sequence breaking are interesting
everything skippable - I'm tired of watching dialogue or seeing the super metroid opening cutscene for the millionth time
edit again: and also a small state space so a full log of states can be stored along with the input file to allow for proper rewinding, even with a video style seeker!