I’ve found that most Case Management requirements can be satisfied by BPMN - we do have a CMMN engine and Modeler but we’ve found that most people don’t need it and so addtions to that part of the engine are few and far between.
I would always suggest that you first try to model your case in BPMN as well as CMMN and see which one works best.
Performance and scale is a very complicated question because it depends on a lot of factors.
This video is from 24 hour fitness, they spoke at CamundaCon a few years ago and they scale like crazy