The only reasonable explanation for such a short lockout timer would be that they want people to give lots of feedback and quickly about the dungeons and how they work so SE can optimize future ones.

If the other dungeons added in 1.19~1.20 are also 5 minutes - Yoshida needs to rethink things. He totes about EverQuest all the time and wants to add things from it. This is not EQesque. This is even more casual than Hello Kitty Online.