We found Hertz to be faster than any of the onsite locations. Their shuttle is always waiting and we are driving away from the lot in less than 10 mins. When you go to leave MCO do curbside check in and then return the car.
Consider joining one of the programs where you can bypass the counter and go straight to the onsite rental car garage - for example Budget Fastbreak, Avis Preferred, National Emerald Club
I use the Budget program and just walk to the kiosk in the garage for my keys. I also use the Dollar program which puts you to the head of any line in the terminal.