Something like this would be my answer as well if I cared. Maneuvering in battle is far different from just cruising around at "I need to get from point A to point B" speed.Take a look at what I did for a cheetah. I think something like that could work for flying creatures.
Feline Speed (2/ Short Rest). When the cheetah takes the Dash action it can move at 10 times its speed (500 ft.) during this movement. Additionally, if the cheetah succeeds on a DC 10 Constitution check at the end of its turn, its ability to use Feline Speed recharges.
BONUS ACTIONS
Feline Agility. The cheetah takes the Dash action.
Basically the speed listed at the top of the stat block is a combat speed, what they can do while engaged in combat. For flying creatures it would probably be a hover speed. I then gave it traits that increase its speed.
I also gave dragons the following trait:
Overland Flight. When a dragon is not engaged in an encounter and has flown in a straight line for at least 2 rounds, it can move at 10 times its flying speed.
Then again, it's an oddly trivial thing to me to care about. If I cast animal messenger and it really matters how fast that hawk can travel, I'll google the flight speed of a hawk. When it comes to creatures that couldn't fly without the aid of magic, I care even less. I don't assume that combat rules apply in all situations.
