Depending on when you were using Scala, it could have to do with abuse of implicits; back in like 2012/2013ish there we a bit of a craze with using implicits everywhere which would make any sort of IDEs job incredibly tough and compilation painfully slow. I write Scala in Emacs at the moment so I can't speak to intellij but Emacs certainly isn't the fastest editor around and I've yet to run into major issues.

The core team also finally realized how important tooling is and recently dedicated a team (a bunch of teams?) to working on language server based intelllisense which is nice.

I'd highly encourage anybody who last used Scala a while ago and didn't enjoy it to give it another shot at some point in the future. Lots of improvements over the past few years (and some really cool stuff coming up with Scala 3) make Scala IMO easily one the best languages around.