The release notes accompanying iTunes version 11.1.3 point out two problems addressed in the update, one being an equalizer fix and another for viewing large content libraries.
From the release notes:
This version of iTunes resolves an issue where the equalizer may not work as expected and improves performance when switching views in large iTunes libraries. This update also includes additional minor bug fixes.
Apple last updated iTunes in October amid a flurry of first-party app refreshes associated with the debut of OS X Mavericks. In that release, the digital media hub gained compatibility with Apple's latest Mac operating system.
iTunes 11.1.3 can be downloaded via Software Update or the Mac App Store.