I have a 2012 Mac mini and experienced similar issues as you.
First, I would definitely take a look at any software related issues, as discussed by others. Hard drive may be full or some other software related issue may be running rampant.
Here was my situation...
When you experience the slowdowns, place your hand on top of the Mac mini chassis. Is it very warm? During normal usage, it should still be relatively cool to the touch. If the unit is running warm, your fan may have silently died. This happened to me. Normally, I am told the fan should sound strange before tanking, but in my case, it just stopped spinning. As a result, the Mac mini starts to thermal throttle when it gets too hot. Luckily, I was able to find a replacement fan for < $12 USD (with tools) on Amazon.
Additionally, if the thermal paste on a 2012 Mac mini hasn't been changed since 2012 or so, it will have the consistency of floral foam at this point and is likely to be ineffective at transferring heat. If you're willing to service the device, I'd recommend a new application of thermal paste.
Finally, heavy Chrome usage should be compensated with 8 or 16GB of RAM, which is fairly inexpensive these days relative to the latest modules.