I find the Multiprocessing Prefs misleading. The caption “Actual CPUs that will be used” should instead read “Number of AE rendering instances”
When using the Multiprocessing settings, AE spawns multiple copies of itself in the background to render. These background render copies can themselves use multiprocessing. If you spawn too many of these, and they are all trying to use more than one processor, they can step on each other’s toes and slow things down.
I have an 8 core hyper-threaded mac pro.
I find it generally works best when I set the maximum ram allocation per CPU,
and restrict it to using no more than the number of real (not virtual) cores – 2.
In my case 6, in your case 10.
Even with these settings, I regularly see it maxing all CPUs when rendering.
Sometimes I only see 6 maxed, it depends on what I’m rendering.
I also use a really handy utility called iStat Menus:
https://bjango.com/apps/istatmenus/
It puts my processor usage bars up in the menu bar.
You should see a steady use of processors when rendering.
If you see your processor use is oscillating between full and zero,
your background processes are stepping on each other’s toes, and
you should reduce the number of processors you are telling AE to use.
Need a quick break from motion graphics?
Try my game Constellation at:
https://www.paperdragongames.com