I know. I'm doing the maths. Also, as a note; a toxic nightshade is the same (flight skill/speed wise) as a Nightfury. however bigger. 
Assuming the standard drag equation of;
Pd = 1/2pv^2CdA.
where PD is drag
P is the mass density of air.
v is velocity reletive to air
A is area facing.
Cd is the drag coefficent

We know that the dragons evolved to fly, so we can assume they have a streamlined body, Thus a coefficient of 0.04 is applicable. The p is the mass density of air is aprox 0.001225 g/cm^3 
V is velocity which is Acceleration by time, Acceleration due to gravity is mass Per second. Its a big dragon, so I'm saying 400 to 500kg. considering a fully grown elephant is 2750kg on average, and the weight of horse rangers from 400 to 600 on average. Thats 400m/s acceleration. So yes. A good sized dragon, from high up, I'm only talking 50 to 80km, which we see the dragons flying around in ALL the time. (thats the average cloud level). This is not taking in to account the extra weight of the rider, Nor the adjustments for atmospheric thinning. However, I could only hit Mach 3, at best. assuming standard atmospheric composition, Otherwise I end up outside the Karman line. (which I'm defining as the upper survivable limit, since in this 'verse they can survive fine flying around at cloud height.)

Yay for SCIENCE.

Additional; This is NOT on the flat in a stright line. This is in a dive, a speed dive, uninterrupted from miles in the air. for any situation, other than a straight line race, this is useless. Val is just pushing limits of flight. as per usual for him.