I did a complete reformat and clean reinstall of macOS Catalina 10.15.5 on mi iMac 5K.
Using the default color profile “iMac” or “Adobe 1998” makes terrible laggy graphical performance in Renoise.
Changing the color profile to “sRGB IEC61966-2.1” makes Renoise to work smoothly.
Even changing color profiles while Renoise is playing makes difference immediately.
I can live with this workaround, using this specific profile for Renoise sessions and the default one for the rest.
This is a quite weird bug, by the way.
Maybe a OS X “Metal” renderer will be considered for upcoming Renoise versions?