Just as another data point, our project is 16k lines (including tests) and I just timed it at 2.7 minutes to run "lein cljsbuild once" with no difference when setting those jvm-opts. This is on a quad core 2.8ghz i7 MBP.
:compiler-stats has been a thing for a while now. This would give more information - like how much time in ClojureScript vs. advanced compilation. Also the compiler version matters - 1.7.170 is what you want to use. Otherwise you may need :recompile-dependents false.
