advance physics only if it's been >= 1/15 of a second since advancing
Since advancing what?
Since the most recent physics update, regardless of what caused that previous update.
else if turbo-fast-forward {
advance physics 9 times in a row
How does this translate to a particular speed, though? Something could be done 9 times in a row but very slowly.
It isn't guaranteed to translate to a fixed real-time speed. The unlikely extreme case would be when a single physics update takes longer than drawing everything to screen.
Most people will manage 60 fps even at turbo-fast-forward, and then it will translate to 9 * 60 = 540 physics updates per second. When you don't manage 60 fps, it's still faster than anything else, and I hope it's sitll fast enough then to not bore anyone.
Singleplayer Lix isn't a dexterity challenge, and the fast-forwards aren't designed to run at a fixed real-time speed. Their speed merely caps at 60 or 540 physics updates per second. If you need to maintain a minimum speed, you'll have to do something else, e.g., frameskipping, than Lix does.
Multiplayer Lix is only runs at normal speed, never at fast-forward. Normal speed in multiplayer features speedup/slowdown to keep the players largely synched with each other, but that's beyond what I explained in the gist.
date & time to do this, I'm sure it would be incredibly useful
This Sunday, February 19th, in the evening? I can manage any of 16:00 UTC, 17:00 UTC, 18:00 UTC, 19:00 UTC, 20:00 UTC.
-- Simon