I am not a frequently flyer like many on this forum but PHL is the airport we fly thru a couple of times a year, always a F to A (and the reverse on return) connection.

I would not chose a 40 minute connection. Carry-on luggage can work against you if you are on a regional jet or Dash 8 because so much has to get gate checked. The ground crew is usually pretty good about getting it to the gate but it is still a delay.

We typically land in PHL between 6:30-7:30am and the buses seem to run smoothly at that time of day but one hiccup and you're in trouble if you have less than 50 minutes. Last time I timed it, it took 25 minutes from my feet hitting the ground at my F gate and the boarding area of my A gate. One too many times my next flight was boarding as I walked up to the gate, that is too close for comfort for me so I look for at least 90 minutes.

The PHL is long and gate location within each terminal can make a difference as well.