Slow Mac mini

I bought a open box Mac mini in mid-2011 from Best Buy. When I read the specs on it it didn't seem bad because all I wanted was a starter computer to use. When I first plugged it in it seemed to take a while to boot up (especially for a new Mac). I have been using it off and on since 2011 and no matter what I do it runs very slowly and lags all the time. Opening a program makes it freeze for a few seconds. While typing, it starts to lag horribly. It is currently running macOS High Sierra 10.13.6. It has a 2.3 GHz Intel Core i5 processor. It has 9 GB of RAM (originally had 2 GB). I have wiped the HD and reinstalled High Sierra several times with no result. I'm not sure if it is hardware or software related. Any help would be appreciated.