The wither caused so much lag it made my world unplayable.

So my boyfriend just tried to fight the wither. The whole game got so laggy he could barely move, let alone fight. Now his whole world is running at about 2fps. He has done so much work on that world and now he can no longer play it beacuse of how laggy the wither has made it. He cant get rid of the wither either because every time he tries to go near it he cannot even move due to the lag. (I'm aware that this should be a story complaint but this damn site wouldn't let me select anything other than bug)