Slow build-steps: ~2-5s per module on cache hit

Created by: bkase

Thanks again, @dreiss for helping with my last issue.

The build is fast now, but there seems to a constant ~18sec overhead whenever buck build app is executed.

I attached traces, but it seems like there isn't much information on cache hit. Let me know if there is some way to produce more verbose profiling information.

clean build trace no-op trace

The gen-rule to hack in kotlin takes almost no time, but the cache hit for the module seems way too long.

I don't quite understand how the [cache] field in .buckconfig is supposed to affect performance, but changing mode to dir under [cache] doesn't seem to noticeably affect build times.

Our build time has already halved, but it would be really nice to shave off another 20s.

Thanks!