Here's the deal: Yes, your metabolism affects your size, and the faster your personal motor runs, the more calories you'll burn. But the truth is, a slow metabolism isn't the real reason you're packing on pounds. You are the reason.
As a general rule, arrive at the airport no later than two hours before your domestic flight is scheduled to depart. This is somewhat later than the typical suggestion of three hours for an international flight.
You're probably OK with 90 minutes. I sometimes arrive at my home airport 60 minutes before a flight because I use the expedited security line and because I know where everything is and how long things usually take. But when I'm flying home from an airport that I'm not as familiar with I leave myself more time.