I have been very happy and lucky with AA's on time performance on my flights recently. I believe the two big reasons are

1) Hubs at terrible airports ORD & JFK are too busy IMO

2) Checked bag fees cause everyone to bring a rollerboard. Every flight the last people to board are encouraged to gate check which causes a delay. Lots of people just can't figure out how to store their bags, and the flight attendants don't help much. On my last flight I didn't want the plane to be delayed and was sitting near the back as I gave the upgrade to my dad when the FA declared that they must gate check the four rollerboards. I immediately remapped 3 luggage bins in about 30 seconds, and had all four bags stowed nicely. No gate check, plane left on time and even landed 5 minutes early ^ I think there is lots of room for improvement and the FA's are certainly not helping the situation on this point.