Generate objects when players is near and destroy them when the player is not.

I'm working on an open world game and it's turn out to be extremely laggy when playing due to all objects rendered at the same time. Don't know why but now i'm trying to find a way to fix it. I'm thinking of using instantiate to clone them to an assigned position but i don't know how to make them only trigger when the camera a.k.a player walks near them. Maybe using sphere cast perhap and tagging the objects and using if - else. Can someone please help me on this? Thank you first. Oh, and i don't really know which topic does this subject fit best so please spare me on this. Thanks.