Control your Mac Fans
Macs are clever - or so we are led to believe. They do everything well with no fuss. Well.... maybe. In the case of cooling, especially in the Mac Mini, they can need some help.
The problem is a thing called thermal throttling. This occurs when the Mac gets too hot and it turns down the speed of the processor to stop it getting out of control. There is a way to help out the Mac and do a better job of making sure thermal throttling never occurs. It's called Macs Fan Contol and can be downloaded here.
For a full explanation of how to use this app, read this article from How to Geek