Huh, you know, I never did make the connection between how no one knew anything was going on until the first on-train zombie attack, despite the fact it got so bad so quickly that Seoul was overrun not all that much farther into the movie. I mean, in context, his trip to Busan was to only eat up the morning as far as his schedule went, so that means that Seoul went from "normal enough that no one was commenting on anything weird" to "everything's collapsed" in a couple hours.
With the end, I did find it annoying how it was obvious the asshole dude was turning and the main guy did nothing until he had finished and was hostile. The "not overpower" didn't overly bug me. He wasn't exactly "Mr. Fitness" himself and was going against an opponent capable of "one-hit kills" due to the contagion aspect of zombie/hate plague bites.