My world stops and my game is not saved, plz help me

Hello! how do i explain my problem, what happens is that when i play everything goes normal suddenly the animals and mobs stop as if nothing but i keep playing fine, when i want to quit it keeps saving the world for hours so i have to open the manager tasks so that java closes completely, when I return to the game it saves me much more behind the place where I was, I thought it was the problem of the mods but I took out several and the problem continues, I can't play like this, they give me some help or advice please, I would really appreciate it. I play in the version 1.16.5