title: String subscript takes long time
body: <p>I read a String from plist file, the string have 105625 chars, which as 325 x 325 grid node information, I use loop to fetch the char:</p>
  
  <pre><code>for x in 1...325{
      for y in 1...325{
          let char = string[string.index(string.startIndex, offsetBy: x * y)]
          ....
      }
  }
  </code></pre>
  
  <p>but it is very slow, why?</p>