I didn't really mind the loftwing's base speed. Maybe because I liked flapping to gain height constantly and then turning slightly downwards to get more speed. That's speed without using the A button and it was enough for me, so you must've seen by now that I disagree with the idea of the boosts being too slow.
That said, I thought the sky-diving was pretty fun, and because of that you bet I always made sure to gain a lot of height even if it wasn't needed. I found a certain enjoyment in that part of the sky travelling that made it all worth it for me, so maybe that's the reason I'm not really bothered by the loftwing's speed. That could also have to do with my opinion of the sky, which I actually found to be really small. I didn't think the exploration aspect was that much exploited in it, which could be another reason I found the loftwing's speed to be just fine.
Then again, while I don't think the loftwing was too slow, I guess my reasons to think so make my opinion somewhat debatable. (yes, even to me)