The time the ball is on the strings is called “dwell time.” This is typically .004–.005 seconds. Softer strings have longer dwell times and stiffer strings shorter.
Longer dwell usually indicates more stringbed deflection and less shock.
The dwell time represents the amount of time available for the strings to exert a force on the ball in order to launch it. Stiffer strings deflect less so they generate higher forces but for a shorter period. Soft strings deflect more, lessening the force of impact (and launch) but does it for a longer time. The net result for ball speed is that a large force applied to the ball for a short time is about equal to a smaller force applied for a longer time.