Edit Simon: I've split this off
Fork: SuperLemmix.
I'd probably want to try and find a way to make it so it's the same speed regardless of what computer you run it on, and at this point I have no clue how this would even be approached. "if Superlemming=true, enable fast-forward" is relatively simple, whereas dealing with frame rates directly I can imagine will be much more difficult.
NeoLemmix sets a frame rate
cap and runs as fast as it can subject to that cap. If the hardware can't manage the full fast forward speed, it'll just run as fast as it can. In extreme cases (slow hardware and/or large levels or with a lot of objects), it might not even manage the 17FPS of normal playback - again, in such a case it'll just run as fast as it can. Superlemming mode, back when it existed, just altered the frame rate caps.
Unless you can find a way to replicate the exact physics (or something you deem close enough and bug-free enough, if you're okay with breaking exact physics) with more efficient code, the only way you're getting around that is by frameskipping (ie: not actually rendering every frame if the hardware can't keep up).