Click on the Apple in the upper left hand corner, choose System Preferences.
On the top row, last icon is Spotlight, click on it.
There is a tab named "Privacy" click on it. This was actually designed to keep spotlight from indexing potentially embarrassing items, but you can use it to prevent indexing useless items as well.
To add directories to the exemption list click on the + button at the bottom of the listbox. Choose what directories to not index and that is it.
Personally I only use iTunes to manage my music, so I have no value in having 30,000 songs indexed. In theory because the index is smaller it should also be faster.
If you have installed Onyx, on the Maintenance item, Rebuild tab, there is a choice to rebuild the spotlight index as well. You might want to try this. Set to exempt your music library from spotlight then run this job early in an evening and let it run all night. A fresh index sometimes helps alot too.
In general, once a month I run everything under the Onyx Automation tab - it's good to clean the cruft out.