Things that will affect speed:
- shorter boards go slower
- extruded bases are slower, sintered bases are faster
- heavier riders go faster
- Unwaxed boards go slower
- As others have mentioned, your riding technique and the line you take will affect things.
- As well as questions of top speed, there's acceleration. If you accelerate to top speed faster you'll get there sooner.
Just for kicks, challenge one of your friends to a straight-bomb race - no turns, no speed checks. Start out from a dead stop. See if he accelerates to top speed more quickly as well as watching for top speed. This will narrow down the possible reasons.
Other tnigs can affect speed too, such as your riding profile. How wide is your silhouette? Are you in a forward stance? Wearing a puffy jacket? Carrying an open umbrella?