Horrible Lag In Singleplayer But Not In Multiplayer

Hi. So I am playing on singleplayer. Whenever I create a world, it lags of course (because of the world generating) then after that, its fine! Then 30 seconds later its unplayable. I dont go to new territories so there is no chunk loading but i start to move slower then eventually I can barely move / freeze. I dont know what to do. Please help!