UPDATE! This is definitely a Big Sur problem. All good after going back to Mojave. I’ve edited this post to make it easy to find.
My MacBook Pro (13-inch, 2017, 2,3 GHz Dual-Core Intel Core i5, 8 GB, MacOS 11.0.1 Big Sur) becomes very hot very quickly running any Renoise project, no matter what the CPU load is. Just after a few minutes the fan is very loud and I start feeling the heat through the keyboard. So I close Renoise and everything goes back to normal right away.
None of it affects audio performance or reflected on the CPU meter (it’s around 3-15% with the latency from 3-20 ms).
Reaper and Reason are running smoothly.