Update: (May 03, 2021) Today, we released our largest drop of games featuring FPS Boost to date, bringing the total titles boosted to 97, many of which are included in Xbox Game Pass and EA Play (included with an Xbox Game Pass Ultimate subscription).
What is FPS Boost?
'Get ready for FPS Boost, higher resolution, and Auto HDR from Xbox One, Xbox 360, and Original Xbox games. Experience game franchises from past generations and enjoy the games you own and love at no additional cost. For supported games, keep your game saves, add-ons, and hard-won achievements. With the power and speed of Xbox Series X|S, your favorite games play better than ever.'
Full List (Title - Series X / Series S / ✔ off by default)