Painter 2019 Mac High Sierra Multicore Not Working

Title says it all. Only one core is being used by Painter, no matter the multicore settings (real watercolor brushes are taking forever to diffuse!). I've used the default of 4 cores (half of my iMac 2017's 8 cores). I've also tried 8 cores. No difference. I've also tried resetting to Painter factory defaults. No help. Painter 2018 works fine with multicores on the same machine. OS version is 10.13.6 (the latest).

Is this happening to anyone else?