Would you mind sharing a similar Reaper and Renoise song that clearly shows this? This could be a good starting point for debugging and improving things.
The new M1s with their drastic enforcement of CPU power saving are also a special case.
Always make sure you only enable as many CPU threads as there are performant (fast) cores in Renoise for Audio. Using low-power cores in mix with fast cores will probably make things worse for real-time audio applications.
We are already making use of Understanding Audio Workgroups | Apple Developer Documentation in Renoise, but maybe Reaper does something else on top as well.
Either way we do need more info and testing here before doing anything.