In Reason 12.2.9 the Max Audio Threads function was introduced. This made it possible to select an optimal CPU core configuration for Reason. With Reason 13.5.1 this function has been refined to also support Intel's new Performance Hybrid Architecture CPUs.
Intel's hybrid CPUs can have up to three different types of cores: Performance cores (P-cores), Efficient cores (E-cores) and Low Power Efficient Cores (LPE-cores). They can also use hyper-threading.
Depending on the CPU core configuration in your specific Intel hybrid processor, you can now select some additional Max Audio Threads alternatives.
The "Default" alternative at the top of the Max Audio Threads list corresponds to one (1) audio thread for each P and E core (but no LPE cores or hyper-threading), minus one (1) audio thread, which is used to ensure that the main audio thread can run efficiently.
The example in the picture below shows a hybrid Intel CPU with 6 P-cores and 4 E-cores (but no LPE-cores), with hyper-threading:
Choosing "Default" in the Max Audio Threads dropdown in the example above would correspond to the "9 (6 P-cores, 3 E-cores)" alternative (1 thread per core, minus 1).
If you are experiencing performance problems, for example when using VSTs in your songs, try selecting another Max Audio Thread number, starting from e.g. "8 (6 P-cores, 2 E-cores)" in the example above and going down one number at a time until you get the best performance.
If you are not using any VSTs in your song you might want to try a higher (>9) Max Audio Threads number to see if this works better. It's mostly "trial and error", so make sure you check out the different alternatives.