Some games after CU Update was stuttering like hell, so I've tried all, disable Game Bar, Game Mode... no fix.

And Microsoft employee answered my post on reddit saying to untick "Show Game Bar on fullscreen games verified by Microsoft" on "Settings > Games > Game Bar" not just disable the first option but that one too.

You guys can see the games there are whitelisted by microsoft here: Your request appears to be from an automated process

and the answer here Windows 10 CU - Fullscreen optimizations? : Windows10

This fixed all my stutters on CU with Overwatch and GTAV.
Hope this helps someone!