There are several options, all leading to solution.
I see in your profile a Formula board...
Here is the problem (I think):
To go fast on formula you give a lot mastfoot pressure, you need really to push the board into the water, by High boom, big sail etcetc. You probaly give automatically a lot down pressure.
To go fast on Isonic you dont need that much pressure. The Isonic goes fastest when you run it on the last suface next to fin. About 20cm from tail.
To reach that:
-Lower boom (bit by bit)
-Always keep maximum loose leech
-Use longer harness lines (like 28-30")
-Front strap in the back possition
-Outhaul deppending on condition and course (I often use negative outhaul)
-Masttrack back possition
Try to make a comftable stance, but the board should be really loose. You even can try to lift your front foot, to let to board go loose. But do it real light in beginning otherwise you can take some air
I am with
Isonic 135 + 7.5 (or 8.1) with 38cm fin and 14 knots of wind is good for me.
Isonic 105 + 6.9 or 6.3 with 34cm fin and 16knots of wind is good for me.
Isonic 135 :::: 55 kmh == 30knots (max peak speed) (only light conditions)
Isonic 105 :::: 72 kmh == 40knots (max peak speed) (some stronger conditions
Here video of my style hope you like it