I will probably wonder how to do this again at some point in the future. So here it goes - this is how to run a command on the efficiency cores on a macOS machine.

  taskpolicy -c background COMMAND

  # to sync my mail, which triggered this to begin with
  taskpolicy -c background mbsync -aq

I haven't done any measurements on mbsync to see if it improves anything or not, but this just Feels Right™ so I wrote it down.