Instant could work, but another idea is that it could take a bit of time to destroy terrain, but pass through air instantly (that might be what it does in L2, tbh - can't remember exactly). It is a laser, which is made of light, so it should travel at the speed of light (basically instant on the scale of Lemmings), but it does have to work its way through the terrain, after all. On the other hand, that could lead to some fiddly timing puzzles, so maybe instant is better (although to be fair, that can already be done with the other skills that also all take time).