Following up on this:
I tried replacing babel-loader with swc-loader, and got it working with the Course Authoring MFE. The build time dropped only from 65s to 60s, which makes it hardly worth the effort. I then went to great pains to strip out all the SCSS from the MFE and disabled PostCSS, and the build time was reduced further from 60s to 48s, which is of course not realistic because we do need to actually compile the SCSS.
This is a far cry from the substantial improvements shown with Vite (though it was measured on a different MFE, so I can’t say it exactly compares).
So my tentative conclusion is: if our latest frontend plans depend on webpack’s module federation, we’re probably going to be stuck with slow builds. If we ultimately take a path forward that doesn’t actually use webpack module federation specifically, I’d really like us to revisit Vite.