I'm in the early stages of this game. Last session was my 12th, so around 15 hours total (various reasons for doing just 1- 1/2 hour sessions).
I beat my previous 500m speed and no crashes, so was feeling happy. Top speed was a bit less, but longer flights.
Changes from previous best in 10th session were:
A bit more experience. Certain things are now automatically one without thinking, leaving me time to look ahead for gusts/lulls an be ready for them.
Slightly less wind
6.5m sail (not 7.5m and wind got up)
Front footstrap was moved 1 hole forward to control nose when going faster, previously when a gust hit the nose lifted and I crashed
Mast foot was 1cm back from before to compensate for foot strap move
Sail was rigged flatter. Previously I had been rigging 1m smaller than for fin windsurfing, but a bit fuller for more power. But in the gusts that resulted in too much sudden power an crashes. So flatter was better.
I was in the harness more and used hips to control power as Jordy's video
In summary, all of the above helped, or maybe just 1 or 2. So many variables.