User blog comment:Miskos3/Fairy Tail 529: Teacher, Review/@comment-30338537-20170402070111/@comment-30462872-20170402111601

Grandeeney was the one closest to lightspeed, I'd say about Mach 5 or above. Besides, they were well off of reaching that kind of speed, although Weisslogia probably could in his unreaped state given he controls light itself.

Don't forget, Igneel tore of Acnologia's arm, and he's said it still hurts from where it was taken from him, although to him that pain would be an 'itch' as he describes. The pain probably debilitates him some, so he can't put his all into flying. Besides, I doubt he sees Wendy, much less Erza or other humans as anything more than a pest, so he's enjoying the chase.

Also, just because he's greater in strength than all the other dragons (which I don't think has been explicitly stated, feel free to correct me), doesn't mean he's the fastest. Granted, he probably is pretty damn quick because of his wingspan, their size and their shape, but I'd say a Sky, Wind or Lightning Dragon, or possibly Shadow or Light Dragons would be swifter than him by a fair amount.