(Late 2013 iMac with 27" screen, Mojave and 24Gb RAM)

While supposedly idle and waiting for me to take a turn EDCE uses about 9.5% System CPU time and in excess of 35% user CPU time. This seems extraordinarily high.

This is an issue with Unity Engine to some degree, and to some code that was not completely optimized for the way Unity Engine behaves in this regard. There was a good amount of effort to reduce this behavior, and i believe there was a good amount of success with the "Experimental" CPU mode.  So if you are having this issue, go to the "Administration" section and select that option (lower left corner of admin screen).

Not a great improvement TBH. That went to a black screen with the % CPU usage at > 100% for a minute or so before I force quit. The activity monitor showed both system and user utilization >35%

Hmm..that's not cool. I ill see if there are Unity reports with MAC in this regard.