Hi. I am not a speedrunner, but I do enjoy watching it; I found a trick in the Cultist Base level, which allows you to fly around the area. This trick can possibly be performed elsewhere, I just happened to stumble upon it here, and figured it might be of interest to someone speedrunning the game. I'll try and describe it as best as I can, given the fact that I did not intend for this to happen, so I didn't exactly jot it all down on paper as it happened; here goes:

In the circular area with four pillars, next to the Slayer Gate, where you can jump down onto some monkeys bars, into a tunnel, which eventually leads to an Empyrean Key (above the whole thing), I managed to die at the exact same time as I grabbed onto a climbable wall (as in, right as I pressed E to grab onto the wall), this resulted in me respawning in the circular arena (with the four pillars). At first, I couldn't see my weapons, but as I started to move, the movement felt floaty, and I noticed I was able to freely around on all the axis (flying, floaty and slowly). Eventually I reached the monkey bars leading into the tunnel again, and as I touched the monkey bar, the weapons became visible again, but I was still freely flying around. That's it really.

I'm currently still at this point, thinking the Slayer Gate will reset it, so I don't currently know how to deactivate it again.

Cheers!

EDIT: The Slayer Gate did not deactivate it. ALSO: You cannot shoot. EDIT x2: Using an Extra Life did not deactivate it either. Actually dying, and reloading from a checkpoint did.