"A complete consistency (either within the compass of the Silmarillion itself or between The Silmarillion and other published writings of my father's) is not to be looked for, and could only be achieved, if at all, at heavy and needless cost."
Christopher Tolkien
The Silmarillian Forward, 1977
It's similar to searching for consistencies amongst any mythology, which is what Tolkien was attempting to create. Tales will always change over time, and they'll always shift focus to what the teller determines is important. As focuses of a society shift, so do the focuses of its related mythology. In this way, I think Tolkien did an excellent job creating a united mythos for England in all the different versions of his legendarium. As the tales evolved, consistencies emerged elements which were formerly key, were discarded, and internal references became more commonplace than external references (see Tolkien's influences from William Morris and Icelandic, Celtic, Germanic, and Anglo Saxon epics)
That was the challenge Christopher noted in the forward to the Silmarillion. J.R.R. had started working it in 1917, and kept making changes right up until his death in 1973.
So he had 56 years worth of papers, and notes, lots of it hand written, to try to kind of reconcile into a single work.
I think he means in terms of time. In order to make it fully consistent, you'd have to have some kind of index and go through line by line making sure everything is saying the same thing.
If I pushed an albatross down a well and attached a crab to it to harrass it on the way down it would also fall, despite being a fantastic winged flier.
Imagine them with wings ill suited to vertical flight and hovering, but very fast in the sky while soaring, and with the endurance to keep going for hours.
It's my headcannon, but I give Gandalf points for forcing the fighter jet into a helicopter arena.
The balrog in Moria was chasing the fellowship. If it could move at 400 mph (by any means) then it would have caught them immediately. We know that some balrogs are weaker than others since it is said that Gothmog was the mightiest. My conclusion is that if the balrogs literally flew by mundane means to Melkor's aid, then the balrog in Moria was particularly weak (and cowardly) and did not participate in rescuing Melkor. I presume that's also why it hid deep underground for so long rather than fighting and being banished along with the other balrogs.
With that said, I think Melkor summoned the balrogs to himself by magical means (but they can't teleport on their own). None of them could go 400 mph. That's just silly. They're not Sonic the Hedgehog. I also think that balrogs can't fly. The word "wings" is a metaphor for the way flames spread from them.
(I don't claim that the text rules out the possibility of wings and flight. The balrog might have fallen with Gandalf because they fought a metaphysical battle, dragged down by the "weight of its sin".)
Edit: I think we actually agree. I'm just elaborating.
Edit 2: I found a picture that shows what I think a balrog's "wings" look like.
In the Second Prophecy of Mandos (present in most versions of Tolkien's Quenta Silmarillion, but omitted from the final Silmarillion due to a perceived incompatibility by Christopher Tolkien), it's mentioned that Melkor will emerge from the door of night after the world and it's powers grow old and weary (the powers being the Ainur, which includes the Balrogs). This indicates that even the gods will grow old and fade as the elves do, which could imply that Durin's Bane is actually just faded and weakened from age and inactivity, since the balrog was sleeping under the mountain for ~5400 years (assuming it participated in the final battle of the war of wrath and hid itself under Khazad-dûm immediately after Melkor's expulsion). Also, there were no more than 3 or 7 balrogs ever according to later writings by Tolkien, which indicates that no balrog was weak or cowardly.
In regards to Melkor's wailing and summoning of the balrogs: Ungoliant ensnared Melkor in Lammoth, which is described as being near the ruins of Angband where the balrogs awaited Melkor's return. It is said that the balrogs rushed swiftly to Melkor's aid, but there is no indication that the wailing was short. In fact, Lammoth is also known as the land of "the Great Echo" where the wailing of Melkor could still be heard ever after, which to me indicates that the wails were prolonged and intense rather than a swift "yelp" followed by a rescue. With this interpretation, it makes sense to me that the balrogs marched quickly, but still took time to rescue Melkor, which would eliminate the possibility of teleportation or sonic the hedgehog running.
Another interesting writing to note is the involvement of balrogs riding flying fire drakes in the Fall of Gondolin. Now, the Fall of Gondolin is definitely an unfinished tale, and was "neglected" more than the other two great tales/lays of Tolkien's later focus, but there are still consistencies between versions that indicate a certainty of specific elements in Tolkien's mind. In each version, there is mention of balrogs riding to battle atop flying fire drakes to assail the city while orcs and trolls attempt to break the walls. This would indicate to me that balrogs could not fly unaided, which would make any wings they have vestigial (unlikely since they are divine), or more a "cloak" of fire.
And completely in keeping with every other facet of fantasy if you want to be that way. A Baalrog can go 400mph just as easily as Gandalf could teleport. All it takes is the stroke of a pen.