In my experience, SBT on the command line is always faster than the IDE, sometimes 5x faster.
A few things that help:
Disconnect "team" (git or svn or whatever) from your project. Somehow the team plugin drastically slows down Eclipse when Scala IDE is enabled. I have no idea why, Its been this way for years. Don't let it auto-detect your source control state and reconnect either. This turns off all of the file / path annotations in the tree that show whether a file is in sync or out of sync with your scource control, etc. (select all projects, go to "team" and "disconnect" -- then change your prefs to not auto-detect).
Change Eclipse to only build when you ask it to (command-B on mac), not on every save / modification (un-check `Build Automatically`).
Those were the two biggest improvements to user responsiveness and compile times for me. On one of my projects, the second one is mandatory or else the IDE is basically unusable, with frequent 1+ minute long unresponsive pauses.