I tested it on my end here, and there is indeed an issue. The mixdown is about half the speed on C12 compared to C11 on top of that the CPU usage is maxed out as well, in C11 it was way lower.
I tested it on a project I am working on at the moment
On C11 it takes about 3 mins to export and CPU usage is around 35%
On C12 it takes nearly 6 mins to export and CPU usage is around 100% on all cores.
This is a serious isse:
C11
C12
