Your Phantom goes faster by tilting more.
When it's fully tilted the sensors are looking downward rather than forwards so Obstacle Avoidance would not work at full speed.
Disabling OA allows the Phantom to go faster. It makes no difference which app you use.
Top speed without OA in P-GPS mode should be about 35 mph +/- the effect of any wind.
In Sport mode you should see about 45 mph.