Even with a rental car, we usually rely on Disney transportation. You've got to go through the park gate, park the car, hop a tram or walk to the entrance and then reverse the process when leaving. I've never held a stopwatch to it, but I'd bet there's little difference in time between driving yourself and leaving the driving to Mickey, and I wouldn't be surprised if Disney is faster.
That said, we didn't have a rental last March and we didn't miss it, we had one last month and used it once during our stay for a grocery stop.
Bill From PA