Author here: Yeah, I was surprised that there doesn't seem to be many options for extra optimizations in the Go compiler. Would be curious if Go experts have more insight or other compiler recommendations.

I doubt its the GC kicking in, but you could run it with the following environment variable set just to ensure that its not doing anything.

    GOGC=-1

    Summary
      ./mainnogc 1000 ran
        1.01 ± 0.06 times faster than ./maingc 1000