Yeah, the UN is pretty notorious for being slow as hell. Also, the movie made it seem like the Avengers would be put under control of the UN Security Council or something similar, and the UNSC has earned a lot of justified criticism over the years for things like the veto right that gives too much power to certain countries. I don't think it's really feasible for 100+ countries to have input in the actions of the Avengers if they expect anything to ever get done, and I've never understood why this argument wasn't addressed more in the movie. (Well, I mean, I do understand, because politics are boring and not what anyone's watching a superhero movie for, but.)