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.

Documenting for posterity since I'm unwilling to do this in runs

In the individual level practice, the time displayed after completing levels is unreliable.

Example 1: 158 frames (from start of fade-in to door entry) = 5.98 seconds

Example 2: 160 frames (from start of fade-in to door entry) = 6.66 seconds

2 frames should of course not make a 0.7 second difference. And this sort of anomaly happens very often in practice mode.

If this is only an issue that affects practice mode, it isn't a big deal. But if the actual in-game timer has the same glitch, then it is a major issue that invalidates all speedrun times.