@Exzap Why you don't use 2 cores?
Or 4 for more performance
no. too much cores can decrease performances
The Wii U has a CPU that has 3 cores. It's GPU contains the graphics cores, an audio processor, and a security and IO processor. In theory, CEMU could spread the work among 6 cores. Since the GPU, audio processor, and IO processor likely aren't actually emulated and only the API calls that access them are, it would probably be a waste to give them each there own core so it would realistically only make sense to use up to 4. Having any additional cores won't improve or decrease performance.
That doesn't mean it's necessarily easy to distribute the emulation work though as instructions still have to be read and translated in a series, before they would are sent out to the different cores.