The clock speed is quite different amongst difference ports of Lemmings. It's known that the Mac has an even slightly slower clock than the DOS version, so what you said makes sense.
The version with the slowest clock we know is Windows, and the versions with the fastest known clock speed are the Amiga, SNES and Sega Genesis (Megadrive).
Anyway, I just confirmed that in DOSBox, if you play the level in high-performance PC mode and use the pause trick, the last lemming will make it out in the nick of time giving you 100%, if you merely dig into the pit and bash. I don't know how you want to count this, but maybe we could call this "2*"? I'll post the DOSBox video soon.
And of course, due to the difficulty of analyzing the level, it's entirely possible there might be a faster solution we overlooked. What we really need is a computer simulation that determines, given the spacing between the basher and a lemming behind the basher, how long would that lemming take to get past the bash tunnel and exit. With this information available, it would then be possible to establish a distribution of lemming spacing that has the desired outcome, or prove that one doesn't exist.