This is frustrating as there is not yet a bug release fixing High Sierra and Mojave issues. It sounds like you’re experiencing the same issue as in this thread: Very low FPS on a 2018 MB pro.
Only seems to be a problem in Mojave. High Sierra has issues with AU plugins, but not low framerate. I assume Mojave would have the AU issues too. Shifting sands seems to be the name of the game with Apple at the moment, what with changing their apis and technologies to fit their aims. (Funny that they name releases after deserts 
) But, also a release for Renoise has been a long time coming.
We’ve been informed some weeks/months ago that development has been resumed, and fixes are in the post…Hope this helps?? Other folks on the forum will have better info for you I’m sure, but unless you can rewind to 10.12 Sierra, then I’m not too sure what to suggest.