I've been playing around a bit and shaved a unit off AKA's test run to give
00:18 on the first stage without a lot of effort. TSC lists 00:17 as the best time, which seems to be the feasible limit without a huge amount of optimisation work.
Comment: the score tally screen is going to get annoying VERY quickly here, I think. :-p
EDIT: AKA, I found a use for your L+R glitch; it's perfect for backtracking after getting the Emerald in UG2 (for 100% runs).
Here's a demo continuing off from my previous WIP; the run in UG2 if completed as a pure run gave 00:18 (clearly not optimised, but that's not what it's meant for at present) and getting the emerald added four units to 00:22.
EDIT 2: Just so I don't feel incomplete, here's the
entire Underground Zone. I want some silly ideas for what I can do during the boss fight if and when I do these levels again. :-)