I can speak from experience working at various events with walk through metal detectors, or even metal wands. No matter how many signs you have and how many times you say "please take all metal objects out of your pockets", a large amount of people don't listen, set the thing off, and the security guard has to take more time to check the person carefully at the point it went off. Keys, phones, cigarette containers, coins. With the large amount of first-time guests and those with metal in their body via hip replacements, knee replacements, and the like that come to Disney, the line is going to be a lot slower in practice than in theory.
I'm on board with the metal detection, it'll make things safer. But they need to at least double if not triple their security check through staff at the entrances or we will see extremely long waits to get through security.