:compiler-stats

Report basic timing measurements on compiler activity. Defaults to false.

:compiler-stats true

Compile cljs.core, elapsed time: 1862.865119 msecs
...
Compile foo.core, elapsed time: 14.719621 msecs
Compile sources, elapsed time: 1879.519163 msecs
...
Optimizing with Google Closure Compiler, elapsed time: 5007.791604 msecs
Optimizing 14 sources, elapsed time: 5123.47005 msecs