mac os catalina late 2014 mac mini

It doesn't seem any different than Mojave, but I've only run it on a test system which has 16GB of memory and an SSD. I know Mojave barely runs in 4GB of memory so 8GB or more of memory is probably recommended. Since you cannot upgrade memory in this system if you are already running Mojave, then I don't expect Catalina to be much different. If you are running Yosemite, then you may notice a slower system unless you have at least 8GB of RAM.

You can always install Catalina to an external USB3 hard drive/SSD to test out system performance. If your Mini uses an internal SSD, then an external hard drive will run considerably slower.

Keep in mind Catalina does not support any 32bit apps. Make sure all your apps can be upgraded to 64bit or uninstall the 32bit apps before upgrading to Catalina.

I didn't notice it being any slower than Mojave on my 2014 with 4GB ram and SSD.