I haven't had experience with newer Intel macs with this issue, so I'm a bit hesitant to try to answer, but here goes. This may not at ALL apply to what you're dealing with.
The problem of incredibly slow behavior with large libraries in iTunes has been an issue since at LEAST 7.0 if not much earlier. I have over 130,000 tracks in my library, stored on a WD 1TB studio drive connected to my "home media server - a G4 Cube" via firewire and shared over ethernet to 3 macs. Do a search for "slow large library" in these forums and you will see NUMEROUS threads all complaining about various issues people seem to have when the number of tracks gets into the tens of thousands, much less over 100k.
In any case, there are some things I've found that seem to help:
1. Keep the actual library (database) files on boot drive. IF you have the music files on a separate drive, it helps to keep the iTunes Library folder on the internal drive with the coverart where it can be accessed faster. This is the database (xml) file that iTunes loads and saves when changes are made to the tags or tracks.
2. Also, if you've not got a lot of playlists to worry about saving (i don't use playlists much, just play cds out of the browser mostly, or have some smart playlists).. it helps to rebuild the iTunes library from scratch at some point. I have no reason to believe this, but I assume it creates a new optimized database this way. (maybe someone can correct me on this... after say months of retagging tracks, playing music, creating and deleting playlists etc... does the database end up 'messier' OR does it correctly rewrite everything and optimize constantly or when you quit itunes?) What I DO know is that if you shorten the path to the music, it will be a smaller file. So don't make it "My Firewire Drive/Music Files/iTunes Files/CDs and MP3s/By Artists/*" this gets written as the path for EACH track in the library file that it then needs to load. Make it "Music/Artists/*" or something as short as you can. Then, what I did was to set iTunes to NOT copy music to the library folder, and starting with a blank iTunes dragged the icon of my shared music folder containing all the mp3 files into the iTunes window. It took overnight and the better part of the next day to read in all 130k files, copy the embedded coverart to the local folder, and the longest part, "determining gapless playback" --which i wish they'd give us a way to disable, takes forever! and i dont' use it (if anyone knows how to stop gapless scanning on a mac, let us know)... but fortunately you only have to do this once.
3. More RAM. My main computer is an MDD Dual 1ghz G4. Until about a month ago I had a gig of ram, then I finally got around to upgrading to 2 gigs. It DEFINATELY made a big difference. Where before I'd see the rainbow ball for 30 seconds whenever I made playlists or changes to tags.. now I often don't see it at all, or certainly much less.
I read not long ago an article talking about WHY iTunes was so bad at scaling up for larger libraries. The author suggested it was due to the library being saved out in an XML database and that unless Apple changes its backend to iTunes, it will not be possible to squeeze much more performance out of it. This probably makes it useless for DJs, radio stations, classical music lovers or anyone with large collections of music with many tracks. I assumed that at some point we'd see Apple rewrite iTunes to take advantage of the SQL database in the OS level, but who knows. I really don't think I understand database stuff at this level.
**Finally... I HAVE noticed that recently it has seemed to be getting more responsive. On one of my other machines (an iMac 800mhz g4!) running iTunes used to be a frustrating proposition with constant spinning beachballs and hangs before it let you pick music or change tracks. Now its actually usable. I don't know when this got 'better' but I think with either 7.6, 7.6.1 or 7.6.2 they have done something to make it more responsive. At least in my experience. So thats a good development.
But seriously.. do a search thru the forums and you'll find a lot more about this problem with large libraries.
Goodluck.
