When launching Minecraft in full screen, MacOS will not switch to Game Mode anymore.
Neither there is an option to open the new “Game Overlay” which was introduced in MacOS Tahoe together with the application “Games”, which won’t even recognize Minecraft as a game in the library.
This lack of Game Mode-compatibility results in noticeably worse performance than before.
It worked perfectly on MacOS Sequoia though.
This is a macOS issue rather than a Minecraft issue, however I have already raised this internally to see if a workaround can’t be found. I will set this to Community Consensus (as others have already mentioned running into this) to ensure that Mojang has a chance to make their thinking on this issue clear to the community.