Many years ago there was confcache¹, that for a time was integrated in to portage via a FEATURES flag for Gentoo users. It wasn't particularly useful in the general case, and never really worked that well in practice. I don't recall the idea really taking off anywhere else, and I can't remember when it finally disappeared in Gentoo either(but it was a looooong time ago).

It was strictly a cache, it didn't run parallel checks or make any other attempts to improve the run time.

¹ The only source I've found right now is https://github.com/fxttr/confcache