Clicking on world, save and quit to title and quit buttons freeze the game for a second

This is not necessarily a bug. Maybe it is your PC? Are you able to run Minecraft smoothly?

It never froze in 1.12.2 and minecraft runs smoothly for me, I think it's an LWJGL issue.

If I would even come to the point of getting into a world. Because my MC crashes when I try to load a world. Might be the generation process aswell.
Going to create an issues cuz of this.

Still seems to be the case, and the game actually dims now when that's happening - odd.

This bug is half fixed because when I click on a world or quit the game, it displays the dirt background for a second.

Not really a bug. Changed the way worlds are saved when leaving a single player game resulting in the "freezing".
To make things clearer, added a "Saving world" message upon exiting a single player game.