Until today it was working. Today when I scroll from one photo to another there is a delay where the photo is pixelated before I can see the actual photo. I have restuarted my computer but that did not work.
Do you have auto analysis turned on? If so, it can take up a lot of system resources and slow down all other operations. If you are on Windows, take a look at Task Manager and see how much CPU activity there is.
Try turning off all media analysis in Edit>Preferences to see whether it makes a difference.
Also, since that behaviour is new, it may be due to external factors, particularly other antivirus or malware programs. I am on Windows with a computer having only 6GB RAM and I have had similar experience. A look at the task manager did reveal the culprits.