Halo Reach, Halo, Halo 2 Anniversary, Halo 3, Halo 3: ODST and Halo 4 will all run at high-end optimized performance on Xbox Series X, with options up to 4K 120FPS in every game.
Today 343 Industries finally confirmed the Master Chief Collection will be significantly optimized on the Series X/S consoles. The massively ambitious Halo: Infinite may be delayed to 2021, but the Series X will basically open up a new dimension of play with classic Halo games running at an ultra-responsive 120FPS at up to 3840 x 2160 resolution. You'll need a TV that supports HDMI 2.1 and 120Hz to take advantage of this feature, though.
No exact details on performance were revealed--i.e. GPU equivalency like The Coalition's Gears 5 that ran at RTX 2080 levels of perf at High settings--but 343i confirms 120FPS will be available in both multiplayer and campaign. The optimizations will of course be free for anyone who already owns a copy of MCC on Xbox One.
The 120FPS updates will release on November 17 on both Game Pass and retail/digital versions of the Master Chief Collection.