title: toDataURL() very slow
body: <p>I have a report with charts and tables.<br>
  I am using the <a href="https://github.com/niklasvh/html2canvas" rel="nofollow noreferrer">html2canvas</a> with <a href="https://github.com/MrRio/jsPDF" rel="nofollow noreferrer">jsPDF</a> to export this report to PDF file.</p>
  
  <p>But the process is taking a long time, more than 11000ms. <br>
  I've tried to change the format, the quality, but nothing worked.</p>
  
  <p>See below the code I used:
  <br></p>
  
  <pre><code>html2canvas($('#first-page'), {
      onrendered: function(canvas) {
          firstPage = canvas.toDataURL('image/jpeg', 0.5);
      },
      background: '#ffffff'
  });
  </code></pre>
  
  <p><br>
  I'm doing something wrong or really is a problem?<br>
  How I can improve the performance?</p>