What sails are you using,
A different sail can also give you a big difference in speed,
Cams / no cams, 1 size smaller in the same conditions, trim of the sail.......
I can foll 4km/u faster with my np speedster 5,2 sail then with duotone e-pace 5,8.
That day i whas 4h on the water in the same conditions.First 2u on de duotone and then 2u on the np.