Problems using my Airport Extreme Base Station with an external HD as an iTunes library
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.