The 4.13.1 release has:

• Some minor performance improvements
• An optional build tag -tags antlr.nomutex which allows a build without
  using mutexes, when the user knows they will not use it with multiple
  go routines. (Note, somewhat experimental)

Note that I did not remove the sort.Slices calls. They will not be experimental for long
and are implemented efficently. Please make a fork if you must use it without this
dependency. Maybe review whether you need to do so for the sake of one call.