If you show up 5 minutes late to a restaurant where you have a reservation will you still be seated or will they cancel your reservation?
Usually you would have a 15minute leeway before they charged you the no-show fee.
You would probably have to wait, and others may be seated before you who were technically booked later but were on time.
If late, they could make you wait or if longer then, as a rule 15 minutes, they could cancel. As you are aware, it is best to show up early. I would do a few things. Rearrange my plans for that day so that I am not going to run late. Another is to call the restaurant to let them know what is going on. (I do the same outside the world to let them know if running late) If it is caused by a ride or attraction breaking down or stopping, ask a CM to call to let them know.