Microsoft Photo Viewer 2010 [better] [TESTED]

Includes a full-screen slideshow mode with adjustable speed (Fast, Medium, Slow).

If you are looking for a fast, simple experience without legacy installers: Microsoft Photos : The default app in Windows 10/11, available via the Microsoft Store microsoft photo viewer 2010

The legendary "ugly but fast" viewer that does everything. Includes a full-screen slideshow mode with adjustable speed