Slllooooooowwwwwwwwwwww?

okay, let's get this out of the way-
After effects is 30 years old this year. it was made before cores were common, it was made before GPU's were a thing, and it was made when 2gb of RAM was huge.

you're trying to run it on 16 cores, with 32 threads. Most of AE runs on 1 core, and 1 thread. this means you're already cutting performance to a fraction of what it should be (good luck getting a modern 2-4 core CPU with the same power as that one though.)
GPU's?... well... let's just say adobe is still figuring them out. 

you have enough RAM! good on you. most people here get an 8-16gb mac you can't upgrade because it claims being a creator platform. you should have no trouble storing the previews that take ages to render

rotobrush is a very slow process. AE is slow, but even resolve has issues with the stuff. It is truly the only part of AE which is slow for an acceptable reason- that it's not only figuring out outlines, but calculating how confident it is in each pixel, and even calculating what colors in each pixel need to be selected and how wide an "edge" should be. in anything above 1080p that's gonna be a crawl. basically, you want maybe two huge cores in your CPU. That's the only real fix I think... well that and nagging adobe to develop that instead of colored keyframes and UI element outlines.

These processes are strictly linear, so the exact opposite is true. Instead of getting moire cores you would reduce the number of cores and get the processor with the highest clock speed to increase per-core efficiency. At this point, though, it's unlikely you will see any significant boosts. Again, simply not how AE works and for that matter the specific code in Rotobrush.

Mylenium

Would you update your graphic card driver and delete the files in the cache disk?

What's "slow"? The Rotobrush itself being an analytic tool that will always process extra frames around the ones you are painting on will inevitably cause some slowness and of course complex shots may take longer just like there is naturally a difference between a HD project and one in 4k. The rest is probably a matter of perception, but you really have to be much more specific about what you think should happen vs. what's actually happening plus provide info about the project, screenshots etc.. Just throwing around generic statements about a tool not meeting your performance expectations isn't useful in any way.

Mylenium