Late 2014 mac mini very unresponsive, fails to wake

I have a late 2014 mac mini (mid-range specs; plenty of RAM) that from the beginning has been soooooo slow. My 2011 macbook air is much faster.

When rebooting, it takes about 10 minutes to be really responsive after reloading Safari, Mail, and a couple other programs. The amount of time I spend starting at the beach ball of death is mind-numbing.

Lately, if I reboot (for an update, say) and then don't log in right away, and let it just go to sleep, it will turn itself "off" or at least into a deep hibernation mode of some sort.

All software is up-to-date. I don't run anything super intensive like Final Cut Pro...

Any recommendations for self-diagnostics? Or should I just make a Genius Bar appointment?

I'm within the 1-year window, but just barely. (I should have dealt with this much sooner, I know, but busy and all...)

Thanks in advance,

-Steve

Some specs: