J.O.
I would wonder about where her comment came from, that isn't typical for a child her age, and discuss the difference between being healthy versus skinny, see if there are any good books out there on this topic.
One other suggestion I have is to not make a big deal about how long it takes her to eat, if she has 20 minutes to eat before she heads off to school, then let her sit there until it is time and when time is up just go to school. I had a slow eater also and it would drive me crazy, if time wasn't a restraint then I would just let them sit there and finish thier food, there were plenty of time she sat at the table alone while the rest of us where done and off doing our thing, eventually she realised we were having more fun so she picked up the speed, she is still the lst one done today but usually by only a few minutes not a half hour.