Speed is one of the three elements of Class and that makes the two inseparable. Speed can be a decisive factor in dirt sprints and uncontested Speed can be a decisive factor at any distance over any surface. Mind you, Speed is one component of Class, the other two being stamina and will.
After all, they're thoroughbreds so we know they can run fast, but for how long? Handicappers want to know which horses can run fast (speed) while demonstrating an ability to sustain the pace required to subdue the competition (stamina).
Moreover, the best horses, those able to effectively calibrate the three elements of Class will be the exemplars with the ability to run fast (speed), sustain the pace (stamina), and respond when challenged by another competitor of equal ability, by exhibiting the required degree of perseverance in the stretch and the stronger will to prevail (determination).
The attribute of speed shares an interdependency with pace
To make things more interesting, since Pace is nothing more than sustained speed, Speed shares an interdependency with Pace as well. Thus in order to effectively evaluate the Speed factor in a thoroughbred horse race, the distinction must be made between Early Speed as measured to the first call, and adjusted final time for the distance as measured by the speed figure; while remaining cognizant of the influence Early Speed exerts over the final time. (Since the faster a racehorse must run at the beginning of the race, the more likely it will expend too much energy and tire in the stretch).
In further analysis, since Early Speed is measured at the first call which occurs at two furlongs in sprints, and since pretty much any thoroughbred horse can go all out for such short distance; then what's important is not how fast a horse can run to the first call but rather, is the horse likely to secure a lead by the first call? Since this has more to do with running style than actual Speed, a vast array of new questions arise carrying the handicapper straight into analysis of the Pace factor - hence the interdependency.