Wow, I never knew they changed any of the release rates and the clock.
I checked it just now. I played the PSP version to get back into Lemmings, so I would had never known. I did played the SNES one first but that was a long time ago. There's always the editor, where you can make levels that are harder than the real ones. I think the reason some of the levels looks smaller from the original like "Call in the bomb squad" is because those terrains that has nothing to do with your objective on reaching the exit makes the level too big for the background. At first I didn't like that when I compared them, but I'm over that.
Edit: I just remembered that you can pause the game before the level begins, and increase the release rate. As for the clock, you can still try to beat the level under it's original time.
Edit2: I went to try and beat "The Far Side" in the PSP under 4 minutes (It really has 6 minutes) and it was harder. What's strange is that in order to beat it under 4:00 minutes, you need to skip the terrain on the left and drop the Lemming all the way down to the terrain at the bottom on the right.
Going to the terrain on the left will only waste too much time.