Heh, I was in this same boat two years ago. Servo used to build its own Rust back then, and that involved building llvm. I hadn't yet discovered ccache[1] and I had an old 4GB RAM laptop with a Core something processor. Took 4-6 hours total IIRC. This was also a really old laptop, so that was understandable. But very very annoying, since sometimes I'd need to rebase over a rustup and then wait hours for the build to work. This was a large part of my impetus to move servo to a state where we download Rust snapshots.

[1]: Btw, if LLVM compile times are getting to you, Use. Ccache. It is amazeballs.