A digital asset management system is likely a better fit than a simple slideshow app. Based on the answers, ResourceSpace is the strongest match: it’s designed for very large image collections, stores links/metadata in a MySQL database rather than relying on one giant folder, and is built for professional-scale archives. That approach should scale much better than lightweight viewers that try to load everything directly.

If your goal is mainly display on a TV rather than desktop management, another answer suggests using a streaming device such as Apple TV, or similar platforms like Roku/Chromecast, with a shared photo stream or screensaver-style slideshow. That’s more of a viewing solution than a cataloging one, but it can work well for casual display.

So: for managing and presenting a million-image library, look first at database-backed photo management tools like ResourceSpace rather than basic slideshow programs.