Just to add to bobbiwoz's comments, the theater seats about 7000, and they have standing room for another 1000-2000. Exiting Fantasmic is a great time for a leisurely stroll through MGM, with some nice nighttime photo ops. But it's never a quick process.
Once out of the park, chances are you'll have to wait for multiple busses. If you drove, you will have to get to your car and then you will be stuck in a traffic jam for 20-30 minutes as EVERYONE tries to exit the park.