Played through all of them, including Gene Gadget Act 1 and beyond thanks to the new patch, so here's a list with my times
GREEN GROVE
Act 1: 0:29
Act 2: 0:48
Boss: 0:44
RUSTY RUIN
Act 1: 1:06
Act 2: 2:03
Boss: 0:56
SPRING STADIUM
Act 1: 1:04
Act 2: 1:36
Boss: 1:17
DIAMOND DUST
Act 1: 1:23
Act 2: 1:10
Boss: 0:55
VOLCANO VALLEY
Act 1: 1:30
Act 2: 0:57
Boss: 0:32 (0:23 if you cheese it*)
GENE GADGET
Act 1: 1:06
Act 2: 0:45
Boss: 1:19
PANIC PUPPET
Act 1: 1:06
Act 2: 1:45
Boss: 2:08
FINAL FIGHT:
5:00
*I should point out that my modus operandi is to go at full speed and get hurt if necessary but still keep going; I say this specifically for the Volcano Valley boss because I stood on the pipes and landed hits as quickly as I could despite getting hurt, hence why I cut down almost 10 seconds from my time playing normally lol. I think rounding up those numbers would work fine for the time attack!