I came across this Field Guide for Better Build Performance by the Slack engineering​ team and inside contained this gem which is mentioned in the monstrous​ Uglify Readme:

It’s not well known, but whitespace removal and symbol mangling accounts for 95% of the size reduction in minified code for most JavaScript – not elaborate code transforms. One can simply disable compress to speed up Uglify builds by 3 to 4 times.

Depending on your needs this can be a quick win for speeding up your build times.