If you place the reservation ahead-of-time, it doesn't take that long at the desk (besides the line, if there is one). Sure, it could be faster, but I've rented plenty of cars from a lot of different companies (with City Rent-A-Car being my preference in SF, and Enterprise everywhere else) and it's pretty tolerable.

Hell, my first ever car rental (to go from DC to Harrisburg because that leg of my flight got cancelled) was a walk-in with Hertz, and the slowest part was picking out a car.

Re: getting to the car in the first place, just about every airport I've been to has a bunch of rental agencies onsite (either walkable or via a shuttle bus), and I suspect HSR stations would be no different.