Hi Sib Shark and from your OP I'd say you have a 420 Sport HP-V.
With our type of floor/hull the air pressure is very important as others have mentioned.
The tube pressures should be .25bar (4psi) and the floor/hull .80bar (11psi).
After launching and the boat has been in the water 10-15 minutes you'll find the air pressures will have dropped off considerably (20-30%) due to the temperature differences which will allow plenty of flex and movement.
Adjust the pressures back to where they should be and you should find an improvement