It’s worth noting that I have tested this on the new version of Catalina (which has various issues currently) and older version of Mac OS.
I have seen performance issues related to running Audacity at full screen or having the UI occupy a large part of the resolution on Mac OS.
I have a filled specced out Imac 2019 and can confirm that this is the case. If I make the Audaicty window size about 25% of the total screen, it runs significantly better with no lag.
If I run it at over 50% of my total screen, Audacity starts using between 40-80% of my total CPU percentage and becomes extremely slow. Even small files with 1 track are very sluggish. I can’t select any of the waveform without having to wait 2-3 seconds for it to update.
Hopefully this helps the team.