The speedrun controls were a last-minute addition, so they're a bit rough, yeah.
The pause button is P because that's how it was in the original.
The fastspin was removed because it's not useful enough anywhere in the game.
The music stuttering is due to the limitations of the Web platform - not the game's optimization, nor your computer. It won't happen on the downloaded version. It might be fixed when we add a custom loading system, but maybe not.
Mario's running speed is the same as the original, I'm pretty sure. The higher framerate makes the whole game feel faster. We removed dashing because it's less useful than diving, and makes precise movement harder.