My copy of "In-A-Gadda-Da-Vida" still plays in 17 minutes 5 seconds, same as it ever did. However, moving around between playlists, and displaying artwork in the Music lib seem zippier than they were in 10.1->10.3.
That's mighty subjective though. To get a firm answer, someone will need to get out a stopwatch and actually measure things.
At first scrolling is a bit jittery when it's loading album artwork. But once it's all there, it's very snappy. The list views are always fast.
But iTunes still doesn't seem to properly cache the album artwork. Every time I relaunch iTunes it needs to load everything again. Pretty annoying, and I can't understand why this hasn't been fixed years ago.
Hands down iTunes 10.4 is faster than any previous version. The speed that artwork loads now is exceptionally faster than before (particularly in the tiled art view and coverflow). Though iTunes still eats up a ton of memory the more images you load into it.