Powered up app throttle control too slow/soft on car?

Hi lego gurus. Hoping someone here can help. My 8yr old son is severely autistic but loves lego. Im trying to connect the Powered up app with custom controls ive created for him, to his Audi Quattro Etron technic car. I can get both the throttle and steering to work when the car is not actually sitting on the ground, so ive built the app correctly as far as thats concerned. The problem is the throttle/speed in fwd/ reverse sems far too soft when its sitting on the ground. The car basically...