Apart from the obvious, the sailor...
If you are reasonably powered, hence the sail size is fine, what are the major factors contributing to the speed?
How much will a type of fin contribute to the speed?
The boards? Sails?
Obelix, I think a fin, board and sail make a difference but given that Spotty has gone 40+ on wave gear with a wave fin, technique/skill and water/wind conditions are clearly the most important factors.
Here is a great technique summary.http://www.iwavesolutions.com/lefebvre/Windsurfing/Articles/need4speed.html
If you want the water/wind conditions... well it's not that far to Sandy Point 
I guess it's a good balance of all factors.
If your only reasonably powered, the sail is too small, to go fast down wind you need to be well powered. (but not overpowered).
Fin will slow you down if it's too big, you don't need much fin going down wind, you only need it big to get back upwind. I've found the biggest problem with the wrong fin foil is ventilation, what holds in well upwind can spin out downwind.
Board size and design is also important, wave boards don't go as fast as speed boards.
Tuning is possibly the biggest factor, getting everything to work together optimally
Thanks guys.
I read the article and your advice and it seems that I need to change quite a few things.
>>>fin will make a HUGE difference !!
This is the fin I mostly use. It's a shortened 36cm JP Weed (down to around 29cm now).
I can see it's not good and should probably get more efficient fin.
It doesn't collect weed, and doesn't slip much, but I'm struggling to get up wind, and as I get close to 25 knts, I lose control unless the water is completely flat.
I should start by getting a speed fin for a 100L board combined to 7m sail. Then apply your advice one at the time.
Thansk RAL INN.
I'm sure the retailers will tell me what the right size is.
What's the performance difference between "Race", "Slalom" , "Speed", "Freeride"...
I've never really got the difference between 'Race' and 'Slalom'.
to me they are both racing and the only difference is the size.
once you get to fins over say 45cm then the racing is course racing.
Slalom racing is smaller gear higher winds (maybe) broad reaching racing.
If you don't have to crank upwind then go for the speed advantage of a smaller "RACE" fin.
this is my brain in action here not giving advice, but rather looking for better clarification.
