For windsurfers it is a must to go fast, as they have no other ways converting wind speed to height, than by pure board speed and a ramp - the wind is almost no help, only a little bit as you can force the board to change direction, thus let the sail lift you up.
Whereas it is "easy" for us, we can redirect the kite even on flat water
But I disagree with you Dimitri - that board speed is working "against" you.
It might be way more difficult if very choppy and windy - in order to set the rail "correct", agree (meaning, for most riders it might be a disadvantage)
But if you redirect the kite sufficiently, and you use your forward speed to head up against the wind on takeoff - then you can always jump higher IMO and also experience
If it is really windy and small kites, the difference is small, agree - and control might make things easier with less speed.
But the lower the wind, the more important board speed (and upwind ability) is for jump height.
The most radical example is a freeride raceboard in 10 knots - it can jump quite okay because of its speed and apparent wind (energy = jump height).
The higher the wind, the less part the board speed plays.
But still a major player, as in 10m/s we still go faster than the wind, or about the same speed, so the contribution to height is enourmous here.
When reaching 15m/s, we might ride over 40km/h where the wind is 54m/s, giving you "67" in terms of height.
If you jumped without board speed at all, you would only have "54" in terms of height...
Not that big difference (+25%), but still there.
If the wind is light, say 10knots (5.2m/s), your forward speed is maybe 36-37km/h (10m/s) on a good board (I have logged these speeds when out).
Wind speed is 18.5km/h or 5.2m/s.
Giving you a height about "11-12".
Without the board speed, the height would be around "5" only.
A difference of +120% !!!
I assume you agree with this, and what you mean is, that for clearing long distances you "dont head up" towards the wind in the same way on takeoff, but keep your forward speed, and even better - increased speed almost on the verge to out of control
But the faster, and in control, the higher the jumps IMO
Although - of course the "loss" when you redirect, means that speed is less important in high winds, and you might jump almost the same with low board speed, thus preferred for control.