title: very tall cell in UITableView cause very slow scroll performance
body: <p>I have a very tall cell with massive text content in it. <a href="http://revealapp.com/" rel="nofollow noreferrer">Reveal</a> shows it's height is <strong>7000+</strong>. When scroll during that cell(it's so tall that will take a while) the tableView's scroll performance is particularly bad. </p>
  
  <p>After <em>Time profiler</em> the tableView scroll. It shows the <code>drawRect:</code> method of one of subviews cost the most. That subview is a third party custom <a href="https://github.com/kishikawakatsumi/SECoreTextView" rel="nofollow noreferrer">textView</a>. I breakpoint the <code>drawRect:</code> method the backtrace like so:
  <img src="https://i.stack.imgur.com/O29uh.png" alt="image"></p>
  
  <p>And it turns out the <code>drawRect:</code> is called many times when scroll during that very tall cell. So what cause the <code>drawRect:</code> is called during scroll inside one cell??</p>