Everyone holds their pee until the movie is over. If you don’t, well you just missed 4 minutes of crucial plot development or missed an epic battle scene. So when the movie ends, there is a mad frenzy to the bathrooms. So the thing I noticed is that instead of waiting in line to the bathroom closest to you, but rather go to the next closest restroom. There are most definitely no lines (since the probability of two movies finishing at the same time is tiny).
Usually the bathroom is on the opposite side of the concession stand (for movie theaters that have the V formation where the pivot of the V is the concessions).
Or you could be the first one out of the movie…
tl;dr Usually there is another bathroom for you to use after the movie that is empty yet relatively close.