RE: Isonic 111 speed
It of course might be the fin and you can of course try thowing money at it (other fins may definitely be faster).
Have you already passed the 30kts with a different board or is this your first attempt to.
If its your first attempt you will have to get used to going that fast and improve over time.
If you already are used to travelling at 30+ Kts I would still give it a few goes before throwing money at a fin.
To be fast YOU have to be fast. Good equipment will make it easier but it is still YOU that makes it happen.