What's the difference between this and the search bar in the start menu?
When the setting is applied the start menu will go back to the old behavior where results appear instantly and always point to your local machine. I'm sure dedicated launchers can do an even better job for deep searches, but personally I've never found myself particularly slowed down by the built-in behavior after disabling that borked web search functionality.
Windows start... I don' t even know if search works or file not found. And is way slower.