Slow loadscreens: It seems that loading music for the levels causes about two to three seconds delay. With the ingame music disabled, loading is almost instantaneous, which is crucial for RTA times.

Keyboard controls: Keys 1, 2, 3 and 4 (not on the numpad) selects the respective Lemming. This does not appear to break with grouped lemmings, so if Lemming 1 and 2 are grouped pressing "1" will select both Lemming 1 and 2. To change how Lemmings are grouped you still need to use your left mouse button. Keys 7, 8, 9 and 0 (not on the numpad) activates ballons the Lemmings are carrying such that 7, 8, 9, 0 corresponds the 1, 2, 3, 4 order of Lemmings. I.e. if Lemming 3 is carrying a balloon, pressing "9" will send him flying with it. It has been observed by some that this function crashed the game so be careful/mindful.

For some general speedrunning tips you can watch the following video. Timestamps:
0:11 Animation cancelling/Traversing terrain 0:53 Trampoline mechanics 2:12 Corner skipping (Wiggling) 3:08 Dead Lemming glitch 3:33 Smart pathing 5:09 Controlling/Moving multiple Lemmings 6:20 Targeting