I suspect (but could be wrong) that there’s very little OS specific optimization anywhere in the code as mono is supposed to take care of that sort of thing automatically (for MacOS and Linux).
Encryption, along with hashing and compression, are known potential bottlenecks (depending on system specs). The latest canary (2.0.3.6) includes some new multi-threading options to help address those areas - but it also introduced some slowdowns so if you were interested in testing I’d suggest you wait until the next release.
Is your Mac sluggish for the entire backup or only certain parts? There are a few parameters to lower CPU and disk IO usage, but generally cutting back on those will also cause the backup to run slower which seems a bit counter to your goal.