Black boxes appear in images in Photos

That is a sign of the image file being damaged.

That, together with the other symptoms you describe suggest your hard drive/ssd might be failing.

First thing to do - if it is possible - is to get a secure backup of all your images (and all other important files). Are they stored anywhere other than in photos?

Have you tried to revert the images with the black blocks to the original version, when the photo is open in Edit mode? Sometimes the previous adjustments applied with iPhoto or Aperture are causing such a blocky experience. I have seen quite a few of such black blocks after migrating my Aperture libraries to photos.

But do check the drive first as Tony recommended. That can never hurt. And making a separate backup of the original image files is always important to protect your library from JPEG corruption caused by a failing drive.

See: Corrupted Originals, the Insidious Danger - How to Protect our Photo Libraries - Apple Community

Good - please get a good backup of all your pictures, and any other files you value onto an external drive.

Then, as a starting point, check the "smart" status of your drive as described here:

https://eshop.macsales.com/blog/45024-here-is-how-to-check-smart-status-of-your-mac-drives/

Have you run any "cleaning", "optimizing", "speed-up" or anti-virus apps on your Mac?