title: Why is NSCell deprecating and why was it introduced in the first place?
body: <p>The "Gradual deprecation of NSCell" section of <a href="https://developer.apple.com/library/mac/releasenotes/AppKit/RN-AppKit/">AppKit Release Notes for OS X v10.10</a> says:</p>
  
  <blockquote>
    <p>Mac OS X 10.10 takes another step towards the eventual deprecation of cells.</p>
  </blockquote>
  
  <p>I heard that <code>NSCell</code> was introduced for performance reason. But then why it is deprecating?</p>