Hello windows10mwp
Thanks for posting Microsoft Community. I'm Marcos Independent Advisor.
Do you want to adjust the viewing spped in Windows Phto Viewer
In Windows Photo Viewer, click/tap on the Play slide show (F11) button > You can right click or press and hold on the slide show to change your slide show settings > and choose slide show speed to slow ,normal or fast.
Let me know if the information helps you or you have another Photo View app
Regards.