Hiding entities when they are not on screen

I think you're missing the point. I know they are not rendered when they're out of range, but still it runs slow because there's collision detection with way too many objects going on. If they're not on screen I don't want there to be any collision detection.

"Hiding them would also skip collision detection, so if you walk backward, you could fall trough the earth right to china."

You're assuming I'm making a first person game then. Which I'm not. :)