what does "single" in "the single fastest way" mean?
"Single" refers to the fact that this way is the only one that fast. To put it in perspective, a car, car A, can be the fastest, but it can also not be alone on that. Another car, car B, can be equally fast. Car A is still the fastest car, so is car B, but neither are the "single most fastest" car.
So, "single" implies it is without equal in its speed.