Well, as @Corrado_Steri said…
Changing WPNAV_SPEED from 3m/seg to 3.1 m/seg SOLVED THE PROBLEM!!!
*Note: wpnav_speed is ment to be set in cm/seg
…there’s something about round numbers that is making wayponits navigation go slower…
And there’s also something about the way you take off as I said before…
Takeoff secuenses (with wpnav_speed=3m/seg) :
Take off in loiter, hover and then switch to auto mode >> wrong navigation speed (slower)
Take off in guided, hover and then switch to auto mode >> wrong navigation speed (slower)
Arm in loiter, switch to auto, 50% thottle to begin the mission >> correct navigation speed!