it takes a while to take a large library, but that's mostly because of intractable problems with ambiguity in metadata searching. I think it's about as fast as is reasonably possible.
My favorite thing about beets is that it doesn't stop processing when it needs user input. Meaning you can tell it to import a bunch of files, completely forget about it, and come back the next day and only give it input for the files that need it.
https://github.com/beetbox/beets
it takes a while to take a large library, but that's mostly because of intractable problems with ambiguity in metadata searching. I think it's about as fast as is reasonably possible.