This is a rewrite of much of how we handle audio rendering threads, to allow smarter use of CPU cores.
The main change is that there is no longer a fixed number of cores/threads used for rendering audio. Reason will now always analyze the song to figure out how many threads can be used. A song with one instrument will typically never need more than one thread.
Previous Reason versions would spin up loads of cores (depending on the preferences), which could consume unnecessary CPU power.
Max Audio Threads preference
The multi-core and hyper-threading checkboxes on the Audio tab in Preferences have been replaced with a "Max Audio Threads" dropdown. Here you can select the maximum number of audio threads Reason should use. It defaults to the total number of physical cores, corresponding to the old "Use multi-core audio rendering" option, but without the "Use hyper-threading audio rendering" option in previous Reason versions. On Apple M1 it defaults to the number of high performance cores.
This preference can also be used to limit the number of threads if you want to leave room for other apps or perhaps multicore VSTs.
This is how the Max Audio Threads dropdown looks for a 4-core Intel CPU, for example:
and here is for an Apple M1 (which doesn't use hyper-threading):
If you are experiencing performance problems in Reason 12.2.9 and later, try selecting another Max Audio Thread number, starting from the highest number and going down one number at a time until you get the best performance.
Tip: If you are using a dual-core Intel CPU and prior to Reason 12.2.9 were using the "Use multi-core audio rendering" and "Use hyper-threading audio rendering" functions, select the "3 (HT)" setting in Reason 12.2.9 and later to get the equivalent performance: