Since OnGUI() is called so frequently, does cluttering OnGUI() slow down the game? what should or should not go into OnGUI()?
Yes, cluttering OnGUI slows down the game at least in the sense that it’s more intensive then putting the same things in Update.
Nothing should go in OnGUI other then the actual GUI. All other things should be calculated at start or happen upon firing events when clicking for instance a button, things like that.