I redid 1-1, 1-2, and 1-3 basically from scratch. The main improvement is a new fighting glitch where moving with precise timing while shooting will shoot more projectiles and basically defeat enemies twice as fast.
This was a surface-level second-pass. There are still probably a few seconds of further improvements, but this is already 04:24.08
ahead of my previous movie
after only three levels. Some further improvement is complicated by the pistons: they block the path, and you can be stuck in a cycle where you're not saving any time on the part of the level prior to the pistons (essentially a frame-rule; although technically, you can probably save time by reducing lag).
I didn't fully apply the projectile glitch to 1-1, but in my testing, it would only save a few seconds because of having to wait numerous seconds for an extremely poor piston cycle, and I preferred the piston damage boost stylistically for now. 1-2 was pretty clean. 1-3 could maybe be faster by damage-boosting up through the toxic waste pipe instead of destroying the computer console to disable it.
One other potential improvement might be turning off the music and/or the sound effects at the beginning of the run. That would probably reduce the amount of lag, with net gains after just a few levels.