do some planning. break your game down into levels and sub-levels.
If you're going to use that object again in this level, then just hide it, otherwise as blanky said, you're going to notice a slowdown when you reload it.
If you need it this level, but you've reached your object limit (very unlikely), EITHER, add a break in to the level, flush all the objects out, and reload just the ones you're going to need. OR flush out your larger objects that aren't required for the rest of the level/sub-level (thing), deleting objects is virtually instant, and substitute them for your new objects (not recommended if your new objects are big, due to the hated slow down)
Most players I would assume would rather spend longer in a loading screen waiting for objects to load than have sluggish performance during the game because you're substituting objects during the level
...maybe one day I'll finish a project
