I first noticed this on level 3 where jumping up the slope rather than running up it seemed to give me more time before having to duck under the fireball. I tested it on level 1 and sure enough, I saved about 0.2 seconds. So, PSA: you can save several tenths of a second for free by jumping up slopes. The main levels where this is relevant are 1 and 3, where combined you can save nearly half a second (in 100% runs).

Conversely, running down slopes seems to be faster than jumping down them. This has a much smaller effect — a few hundredths rather than tenths — and is relevant in levels 1, 2 and 3. I used this to save another couple frames on level 1.

I put together a few strats I found that help with speedruns. Let me know if you find any more.

Level 3: Jump onto the gargoyle then up and to the right to get out of bounds, skipping most of the level and the need for a key.

Level 3 100%: Jump high enough to collect the orb, but low enough to stay below the platform.

Level 4: Damage boosting on the mummy gets you the extra height needed to make the jump. Hit the right side of the mummy to get boosted towards the exit.

Level 8: Double jump from the moving platform to collect the key from below, skipping the upper path.

Level 9 Orb: This one is just a proof of concept to show how damage boosting can be used in tandem with double jumping for extra height; it is not actually valid for 100% runs as it involves taking damage. Hopefully this tech finds another use elsewhere in the run. Jump once, damage boost on the falling slime for extra height, then use your double jump to collect the orb, skipping the upper platform. Make sure to hit the right side of the slime to get boosted towards the orb.