I would define the speed of a race driver as the measure of the time taken to cover a distance. In the same car Vettel tends to keep his right foot planted to the floor for a longer amount of time than Webber over the same distance and is therefore a 'faster' driver
So for a given car the driver who can extract the most performance, take corners fastest, remain at the performance limit longest, and backs off the accelerator least is the fastest
This is a different proposition to being the driver who can get the car around an obstacle course the quickest. This type of driver who can get around a track say with the least mistakes or with the least deviation from a delta or conserving his equipment the best might win races by being the quickest around a track but is not necessarily the fastest driver outright. For me that sheer speed is correlated to bravery, the ability to drive as fast as the car can go for as much of the lap as one dares is the most highly coveted attribute of the race driver and is why they get the big paycheck
So to cut a long story short, I think that nowadays the playing field is somewhat levelled in this respect. The fastest and bravest drivers are no longer able to use this as a trump card as the current regime does not reward this ability. I think this is what MS has alluded to recently, those drivers less able or willing to max a car no longer have any disadvantage as there is no longer any advantage to maxing a car as much as ones nerve can hold