Spinning Rainbow Circle

Back up your Mac if you have not done so already. To learn how to do that please read https://www.apple.com/support/backup/. To learn how to use Time Machine read Use Time Machine to back up or restore your Mac. The reason for creating a backup is that the symptoms you describe could be indicative of an impending hard disk drive failure, so securing your data is a primary concern.

Then, shut down your Mac and restart it in "Safe Mode". To learn how to do that please read the instructions described in Try safe mode if your Mac doesn't finish starting up. Compare how your Mac works in Safe Mode to how it has been, and describe your observations in a reply to this Discussion.

Then, restart normally. If the same problem returns, please read the section If an issue doesn't happen in safe mode for Apple's recommended actions.

Finally, you can temporarily create a new User Account, and log in to that account to determine if the problem also exists when logged in to that Account. To learn how to do that read How to test an issue in another user account on your Mac. When you are finished troubleshooting, you can remove the temporary User Account by following these instructions: macOS Sierra: Delete a user or group. Before removing it, confirm you don't need any of the files you might have created in that Account.

Let me know what you determine.

And is there a way to go back to the previous OS?

Yes, but you would need to completely erase the Mac first. It's better to fix what's wrong.

Not really. Since Sierra introduced a completely new logging system, I do not know of a reliable way to diagnose potential hard disk problems. Until I come across a disk in a state of failure yet operational enough to test I can't provide a means of extracting any applicable log entries that could definitively show those failures. A definitive conclusion could be drawn by installing Sierra on an external hard disk drive, booting from it, and determining if the symptoms are alleviated while running Sierra from that startup disk. To do that read How to set up and use an external Mac startup disk - Apple Support.

Insufficient memory can also cause those symptoms. It's very easy to determine if your Mac's performance is being limited by available memory. Please read Use Activity Monitor on your Mac - Apple Support. If the "Memory Pressure" graph is frequently "red" your Mac will benefit from additional memory. If not, it won't.

Taken as a whole, the symptoms you describe strongly suggest a failed hard disk drive, assuming your Mac uses one of them and not solid state memory. If that's the case downgrading won't help.

Hard disk drives can be replaced. What model Mac is giving you trouble?

I'd give it much longer than an hour, but if it fails to progress after several hours then it's not likely to ever finish.

As I explained earlier, diagnosing a failed hard disk drive is often accomplished anecdotally, and that one is exhibiting typical signs of failure. You can rule out other hardware causes by installing Sierra on an external device as I wrote. If it boots and runs well whereas the other one does not, then that's all the justification you need to replace the internal one, or have Apple do it: Contact Support.

The smallest time increment used by the Mac's operating system is the microsecond. The largest time increment is "Time Remaining".

Let it finish.

Replacing a Mini's hard disk drive is a tedious procedure, but is not expensive if you want to do it yourself. Refer to https://www.ifixit.com/Guide/Mac+Mini+Late+2012+Hard+Drive+Replacement/11716.

It might be, probably is, but other than the expense of time and / or money as long as you have a Time Machine or equivalent backup the inevitable hard disk failure is a non-event.