My Mini (2014) takes for ever to boot up, at least 20 minutes before I can use it....
When it’s up to speed it’s fine all day long,
So it sounds like your mini isn't slow at all...it's just slow to boot up.
I doubt if any of the recent advice that you've read here in these forums recently about slow running Macs is applicable to your situation.
My experience is that when a Mac is taking a ridiculously long time to boot up, but it is otherwise running just fine, the problem is usually that it is encountering an error of some type during the boot up process, that it then times out, and it doesn't actually resume the boot up process until it resets.
If your Mac is booting up with no unusual tones or error messages, your hardware, including your RAM, are likely just fine.
Temporarily turn off all logon items:
System Preferences --> Users & Groups --> Logon Items
Use the little "minus sign" box to delete each logon item one at a time.
You can also go to each app with a dot under it in the Dock, that is open after you startup, right click on it, and uncheck "Open At Login"
Restart your Mac and see if things are are better. If they are, you need to update the offending app.
Next is, when you shut down your Mac, a window will appear and one of the choices will be to "Reopen Windows When Logging Back In". Disable that.
Did you once have an external hard drive, or other network device, or other peripheral attached to your Mac, and now you don't have it attached anymore? It may be that your Mac is still expecting to connect to it on startup, and it times out while looking for it. Go into System Preferences, find the driver setting for that device, and delete it.
One or all, of the above should return your mini to booting as quickly as it did when new.
If it doesn't, let me know. At that point we should do a Hardware Test, and if that comes out clean, a clean re-install of your OS using Mac OS Recovery is probably in order.