Not knowing much about your model (or new compounds for that matter), but anytime you work with mechanical stuff like that you will find that there is a point for optimum performance.
Anything on the string will slow it down, silencers, nock points, etc..
Heavier strings will slow it down....
Poundage (weight) does not necessarily transfer to speed, try adjusting it down to see if it changes.
Lighter arrows will increase speed also....