The five hardest time-saving strats in the game (according to me):

Level 9: time save: 0.10 seconds

Difficulty: 7/10 (tight timing on first two jumps, to beat the projectile)

Level 17:

Maximum time save: 0.20 seconds

Difficulty: 6/10 + RNG?? (The timing is tight to dodge lava. You can't be too fast or too slow.

But the main challenge is fast-falling onto the ramp to gain speed. I'm not entirely sure why it works sometimes and not others.)

Level 20: time save: ~0.15 seconds

Difficulty: 9/10 (basically a blind wall-run + side jump. Must be frame-perfect to not hit spikes)

Level 25:

Maximum time save: ~0.10 seconds

Difficulty: 8/10 (Surviving isn't too hard. But actually saving time requires very accurate execution).

Level 16 (still the hardest) time save: 0.75+ seconds!

Difficulty: ∞/10

The pixel-perfect entry into the tiny gap is somehow the easy part. The side-jump to exit feels almost impossible, or maybe random. Even if you hit the jump timing, you need to hit the shortest of short-hops in order to not fly over the exit door. Trying to do this strat in a run would be a quick ticket to the insane asylum.

New Super-Strat: Level 29 Perfect Cycle

Maximum time save: 0.40 seconds

Difficulty: 10/10

This is the hardest "fair" strat in the game. Unlike Level 16, which is too insanely precise for any human to ever master, this could be made semi-consistent with a ton of practice.

You have to nail every tiny optimization, or you can't make it over the projectile at the top (hence "cycle" refers to projectile cycle).