meow: Please see Multi Core FAQ
Renoise has to split your song into multiple components that then can be spread to the processors - these components are “tracks” in Renoise.
If you for example have a song with 4 tracks running on 4 CPUs, then the 4 tracks will be distributed to the 4 cores (one track per core at once). But if one of the track has a CPU usage of 20%, while the others have a usage of 1%, theres not much speedup you can get form multiple cores. The more tracks you use, the bigger your songs are the better the load will be balanced though…
This has nothing to do with multi cores, but yes, this indeed flickers more than in 2.1. I already try to find out why exactly…