Assuming everything is up-to-date, it might just be defective. It’s rare, but can happen. Thankfully the RMA process is straightforward from what I hear.

Might be worth popping the back cover off and reseating the fan cable, just in case it got loose.

If the fan turns out to be bad, you can pick up a replacement pretty cheap from iFixit.

thank you for contacting tech support, have you tried turning it off and on again?

The reality is that CPU rendering usually doesn’t really scale with more cores, as the bottlenecks lie elsewhere. llvmpipe, for example, is actually limited to a certain number of cores, and although you could always edit the code, you are unlikely to see that much of a performance uplift.