What's causing scene to slow down?

When you instantiate, you're creating an object from memory. when you destroy it, you arent actually destroying it from memory, you're just flagging it that you dont need it anymore, the garbage collect is what actually frees up the memory. If you are constantly creating and destroying every update then your heap memory is going to go up and up, and the garbage collector is probably not going to run on it's own until you use up your allocated memory, which is going to start causing your game to lag.

Thanks for that info - very helpful. Should I try garbage collecting manually? I think the way to go is with instantiating everything at the start and then shuffling the deck so to speak.

Yeah you could try calling it manually, but that wont guarantee a collection. Calling the GC manually just acts as a suggestion that it should collect. I agree that the way to go would probably be to instance everything at the start and rearrange it. In my own projects I almost never have to manually call the GC and just let it run automatically when a new level is loaded

I agree, instantiate/destroy seems to cause a lot of problems speed-wise. I figure I will instance everything on the start and then as objects fall behind the player I will place them upfront again just out of view and change their attributes/positioning etc. That way I get a constant speed rate throughout the game.