In Windows 10, Microsoft decided to make a new app called …Photos…open pictures by default. Unfortuneately it is sometimes very slow to open with some upgraded computers.
If all you want to do is view your photos quickly, it makes sense to associate photo files (like jpeg or png) with a smaller, faster program.
Microsoft does sometimes do something right, and in Windows 7 , pictures used to open with a fast little program called… Photo Viewer. You can still use this program by enabling it in your Windows 10.