Thanks for the prompt answer! (And then I take a week to say thanks…)
I see now some of that was also mentioned on the download page, though that isn’t accessible from the Itch app. And thanks to the mention of the Steam versions, I was able to find this from the Technical FAQ, for anyone interested:
What is the mkxp version and how do I opt into it?
The mkxp version runs Jimmy through a C interpreter. Basically, this makes things run at a much nicer framerate. However, battle music doesn’t loop correctly, and some people experience other glitches, so I felt it was too unstable to have as the main download.