Servo speed refers to the transit time of the servo. It's usually captured with the number of seconds it takes for that servo to rotate 60 degrees. Since most servos have a finite amount of rotation before it reaches its mechanical stop(s), an abbreviated range was chosen so that servos, no matter the max amount of rotation, could be compared more easily. A typical servo speed would be 0.15sec/60°. This means that the servo will rotate 60 degrees in 0.15 seconds. If the servo has a max rotation of 180 degrees, you can easily derive the transit time for the maximum amount of rotation (180/60=3 therefore it will take 0.45 seconds (3 x 0.15 seconds). This unit does, on occasion, get transferred to continuous rotation servos even though RPM would be a more easily understandable unit of measurement for most. To convert from sec/60° to RPM, simply multiply the transit time by 6 (to get from seconds per 60 degrees to seconds per 360 degrees) and then divide 60 seconds by that number. In the case of a continuous rotation servo with a speed of 0.15sec/60°, 0.15 x 6 = 0.9 seconds per 360 degrees and 60 seconds / 0.9 seconds = 67RPM.