Even Photos App doesn't work properly and spies on you, sending your child porn images to law enforcement:
https://social.technet.microsoft.com/Forums/en-US/978e2e07-6b52-43b2-83fc-04ea0f4f141b/windows-10-photo-viewer-slow-to-open?forum=win10itprogeneral"My old windows phone opens pictures better than this."
"The Windows 10 photo viewer seems to be quite advanced and is not your run-of-the-mill image viewer. Mine was slow too, which led me to do some minor research.
1) The app "phones-home" to Microsoft with some sort of usage data. This uses your network connection. If you have a slow network, or other reasons why networking slow, this could be your problem.
2) The app maintains a database (SQLite) of images. The database contains information on all your images. So if you open up the app in a folder with a lot of other images, you will see a slowdown because it's doing a lot of querying (probably to speed up slideshow).
This is all I found so far. These factors explain enough of why the old image viewer would be quicker."