5 minutes sounds like way too much, unless you mean a fresh compile. But then you shouldn’t need to wait that long between changes, since incremental compilation should kick in then.
And like sure you can break shit up into crates to speed up compilation, but to do that you already have to have a design you’re happy with and that’s stable.
I mean, if you have your modules structured in a tree structure and with proper visibility, then it isn’t a particularly big leap to put it into a separate crate. You just move the files, maybe fix some visibility modifiers still, and then a bit of boilerplate to add it to the workspace.
It’s only really when you’re publishing to crates.io, that you don’t particularly want to keep changing the names/scopes of the crates, as they’ll stick around on there for the foreseeable future.
100k lines and you don’t yet have a design that’s mostly settled?