Interesting that the difference between first and second serves was similar no matter what the level of the player. One of the reasons my serve is so effective is the multiple speeds at which I hit. I pride myself on how slowly I can hit an effective serve - almost all the energy going into spin. Combining that with the flat heater and the many other types of serves I hit is very effective.
Mixing up speeds can be as useful a tool as mixing up placement and spin.