When a dragon flies while overburdened or pulling a cargo disk, it looks ridiculous. They're barely moving their wings, which strains plausibility past its breaking point.
It's been established in the lore (I can't remember where...I'd love to see the source again, if anyone knows where it is) that Istarian dragons fly with the aid of inherent magic.
So, my suggestion is one or both of the following:
A) Remove the effect speed has on the flapping animation. This would fix the ridiculousness at high speeds, too, and would cause much rejoicing.
B) When a dragon is flying while overburdened or flying while pulling a cargo disk, a subtle magic particle effect around their wings is activated. The slower they're moving, the more intense the effect.