The PSX Version of MVC1 doesn't have the tag team mechanic, but it does contain infinite assists or the option for your partner helping you in supers.
It's the version I played as a kid but mechanics other than those are mostly similar to the arcade game. I optimized Ryu strats from the arcade speedruns further and managed to get 3 to 4 ingame second fights.
I start with Ryu transformed to use the best version of the air Tatsu, some characters block or have different weights and sizes and are affected differently wasting time in general. I couldn't think of other multi hit unscaled strats to try out.
A better partner could probably save 1 or 2 ingame seconds against Onslaught but i had Juggernaut in case RNG was really bad( it was really bad after all, but I didn't seem to need it) and avoiding Jin and other armor abuser in this ladder would save a few more ingame seconds.