Been riding a 1950 cm2 front wing for a while, reaching Max 15 knots. Now Testing a 1450 cm2 wing and been able to reach 17.5 knots.
What is the key to reach higher speed ?

Hi Gizki,

as motogon said, go for even smaller wings.

But first question should be: Why do You want to go faster, e.g. higher cruising -> traveling speed or take off speed -> jumping hight?

Depending on that you may also check how your feet and leg position is at higher speed.
If you are looking for higher medium cruising speed, both your legs should be close to straight and all your body in a compfortable position. If you trying to go high speed with the boards nose pitching down and your back leg hardly angled you wont be able to hold that speed for long. If it is like that you can use a shim plate inbetween board and foil for example.

But as said, wingers here who ride since 2 years are scaling their foils down. I don't use 2000 - 1400 cm² anymore but since nearly one year a 1000 cm² is my biggest and already tried less than 600 cm² and bought a one with a bit more than 600 cm² for jumping, but I am just 62 kg.

Important when riding small wings is not just only good power but a board that glides well by itself without channels that are to sticky and so on.