Slow start

Here is a list of things that can cause slow start of One Photo Viewer.

1. Most likely if you install One Photo Viewer from Microsoft Store there is some MS Store background service that gets messed up after a Windows update and starts slowly. This is the most likely cause of slow startup. If this is the problem it might disappear after a while, else you can try reset of MS Store app, see this thewindowsclub.com/reset-windows-store-cache. Also try repair or reset One Photo Viewer see thewindowsclub.com/reset-windows-store-apps-windows-10
Else you can try the legacy/Windows 7 installer (and uninstall MS Store version) https://onephotoviewer.com/download/

2. If you have turned on "Sort images as in Explorer" try turn it off, in right-click menu -> "More Settings".

3. Images on a web storage like OneDive or Google drive or network drive might have to download images before showing them, this take time.

4. Windows file Explorer can be laggy. Try start One Photo Vewer from start menu to se if this is the problem.