It sounds like you accidentally decreased your fly speed to the minimum of 50 cm/s, which is the same speed as walking while encumbered with the Quickfooted perk.
The base fly speed can be 50 to 8000 cm/s. Sprint-flying is always 1500 cm/s regardless what you’ve set your base fly speed to.
You say you are aware of the speed setting, but to clarify, are you speaking of these settings?
Even if you are aware of them, you should check them to see if they accidentally were changed and hit whatever you have bound for the Increase Fly Speed button several times to increase your base fly speed back up again.