First boot into Safe Mode and try the library from there. Reboot normally and test again.
NOTE: Safe Mode boot can take up to 10 minutes as it's doing some system cache cleaning, volume verifying and directory repairing.
As a test launch Photos with the Option key held down and try to create a new, test library. Import some photos and test to see if the same problem persists. Does it? (This tells us if the problem is limited to your current library or is more wide spread)
If it doesn't then open your original library again and see if there's any change. If not continue.....
Log into another user account on your Mac, repeat the test library and see if the problem persists or not. This will tell us if the problem is limited to your user account or is system wide.