Roughly speaking, for rapid building at RR 99, 1 out of every 3 lemmings does the actual constructions, while the other 2 would've caught up to the builder before he has even laid down the first brick, and therefore must be delayed or otherwise handled.
I've finally gotten 100% on Tricky 22 at RR 99! The key is to get around the rough observation above, as the solution uses up every skill except the bombers. Don't read on if you absolutely want to figure out everything yourself, but otherwise I don't feel what follows is that much a spoiler to most:
[edit: turns out even w/o using the technique below, it's still not strictly 1 out of every 3 (it's more like 2 out of 5) when it comes to rapid-building across a gap. Just try the rapid-building without delaying any lemmings with any extra building, and observe which lemmings actually fall off during the rapid-building. So it might even be possible to end up with spare builders, or to do things slightly easier than the posted solution]
I said "roughly speaking" originally, because it's possible to influence the spacing of later lemmings out of the entrance to do a little better than strictly 1 out of every 3: as they fall off a bridge-in-progress built by an earlier lemming, some get slightly more spaced out (by 1 extra pixel) because the bridge got higher between one lemming and the next, with the corresponding change in the fall distance providing the extra delay and spacing out every now and then (the earliest being a change from 3 to 4 pixels of fall).
That makes a big difference because 2 lemmings need to be spaced at least 9 pixels apart in order for the back lemming to not catch up to the front builder before a brick is even laid down. Whereas RR 99 spaces the lemmings 4 pixels apart, so every other lemming would be 8 pixels apart, just 1 short! And that's where the insight above can come into play to make up for the shortage without having to strictly do 1 out of every 3.
Timing is key, so even the delay builders at the start of the solution are actually somewhat carefully planned, to ensure they will correctly add the needed extra spacing between certain later lemmings.