I don't know what the avg speed is for a storm but a couple of years ago (can't remember when or the name) there was a storm that hauled ass by us. I remember waking up the day it was off the coast and not seeing any surf, but it showed up the next day. I guess it acted like a boat wake
the slower the system moves the more swell is generated in all directions. When the storms move fast, they direct most of the swell only in the direction the system is moving, and it is generally a shorter period of swell that hits the coast.