Unity builds are a big win in Unreal Engine and basically required w/Unreal, the actual win is surprising large.
EDIT: I'm sure lots of work has been done, not trying to degrade that. Just sharing my experience on my projects, never worked with Chrome.
So, lots of work has been done to deal with the build times. And probably not a lot of low hanging fruit to be found. But, still more work is being done. Support is being added for 'jumbo' builds (aka unity builds, where multiple translation units are #included into one) which is helping a bit with compile and link times.