I don't find book import all that slow. But what I have wondered about is why batch metadata updates are so much slower than single-book metadata updates.
Say, correcting a title or author sort for a few books. Do it 1 book at a time, and they take less than a second each. Do even two at a time (thus using the batch metadata update screen) and it takes far longer than (N x <1s).
I can't really explain that one.