How often do TN Visa applicants miss their flight due to the application processing time?  For me personally, I missed it twice before I finally learned to give myself extra time. The worse case scenario is 2 hours so add this to the time you think it takes for you to go through security line, and walk/shuttle to the gate.

If you are arriving into an US airport that have the boder/customs such as JFK, then that is a different story but then you have other things to worry about if your TN application is rejected.  Never happened to me before but that would be an interesting scenario and something maybe we should blog about one day.

Good luck and give yourself extra time!