have you turned off win10 game mode - that helped me on halo 1 etcI was getting annoying frame drop issues on the New Mombasa levels last night, so i don't think they have solved it.
Using the tab button to go between the old and new graphics i didn't realise just how "lacking", for want of a better word, the original Xbox games was in some areas, especially textures. It didn't seem that bad at the some when i play it back in 2004.