More info in the verge's article
Microsoft doesn’t load the full Windows desktop or a bunch of background processes in this full-screen Xbox experience, putting Windows firmly in the background and freeing up more memory for games. Instead, you launch straight into the Xbox PC app, which includes all of your PC games from the Microsoft Store, Battle.net, and what Microsoft calls “other leading storefronts.”
This was patched, and the vast majority were just fetching thumbnails.