Are you combining the effect with instances of Adobe Immersive plug-ins? Mocha can run w/out GPU enabled, but generally this issue points to the Adobe plug-ins they acquired from Mettle.
Have you updated to latest Adobe 2019 and Mocha 2019?
Generally Mocha should simply fall back to non GPU if the system is taxed, but “I believe” the Adobe internal VR filters do not have the CPU fallback built in and require more GPU.
In Premiere, I have also seen “nesting” effects as a way to get around the message OR last resort pre-renders.