Does iTunes 12 work with Mountain Lion

You've done the most important optimization available by placing the database files (the iTunes library files) on the local drive. If you had that on slow storage (and Airport Extreme is in the slow category as far as network storage) then I could see you having slow launch times.

Even a library with 70,000 items stored in it should only have an iTunes Music Library.xml file that takes about 120 MB of space which should load in less than 10 seconds unless your PC is really bogged down with other tasks on the CPU.

The actual size of all the media doesn't affect the application launch time as it only has to parse the library files and not check each and every file before you can start using the program.

I would look at tuning up the PC or speeding up the filesystem where the library files are and see if that results in better app performance.

As best as I've been able to tell, this feature has been removed.

There are two alternatives to the Lion-provided notification mechanism that provide a few more features and customizability.

If you have Growl installed, you can use GrowlTunes to provide Growl notifications on track and playback changes.

The official Last.fm Scrobbler app provides notifications via Growl as well, the bonus being that it's worth using if you also scrobble your playback history to Last.fm. If you don't you probably want to stick with GrowlTunes.

Growl is a generic notification system with a large amount of customizability, like the ability to customize the alert style. Growl has existed for OS X for as long as I can remember, years and years pre-dating even Tiger, iOS, and the Notification Center as well.