Dipping below the magical 1 second page load time

A couple of weeks ago I started looking into data-uri’s as a way to further optimize the performance of this personal playground of mine. Testing was easy enough; Autoptimize, the javascript/ css/ html opitimizing plugin I now use in conjunction with WP Super Cache, comes with support for data-uri’s in CSS and switching that option on indeed immediately resulted in less requests being made.

While testing I did find a small bug in Autoptimize (in /wp-content/autoptimize/classes/autoptimizeStyles.php) which caused jpeg images not to be taken into account. The regular expression in the code was ‘jpe?j’, while just below in a switch/case block ‘jpej’ and ‘jpg’ were referenced, resulting in neither ‘.jpg’ nor .’jpeg’ files ever being turned into data-uri’s. While reading the code I also noticed that the upper filesize limit for images to be turned into data-uri’s was set at 5120 bytes, but as base64-encoding does come with overhead, I decided to lower that limit to 2560 bytes.

So I made some minor changes in autoptimizeStyles.php on lines 207 and following:

if($path != false && preg_match('#\.(jpe?g|png|gif|bmp)$#',$path) && file_exists($path) && is_readable($path) && filesize($path) <= 2560) {
   switch(end(explode('.',$path))) {
      case 'jpeg':
         $dataurihead = 'data:image/jpeg;base64,';
      case 'jpg':
         $dataurihead = 'data:image/jpeg;base64,';
      case 'gif':

I also  switched my “subscribe” widget from a paragraph- to a bulleted list-approach with the rss- and mail-icons as background images and defined the adfreeblog and creative commons badges as background-images as well. And then I was ready to test the real impact of using data-uri’s on webpagetest.org. Behold the results for my current blog homepage (i.e. the one just before this post got published) with and without the use of data-uri’s (but excluding those dog-slow calls to stats.wordpress.com):

No data-uri’sWith data-uri’s
webpagetest results URLhttp://www.webpagetest.org/result/121106_12_6FZ/http://www.webpagetest.org/result/121106_4H_71R/
#requests (full page)2619
Bytes in (full page)116KB114KB
Start render (median)0.615s0.634s
Doc Complete time (median)0.969s0.870s
Full page load time (median)1.932s1.332s

Let it be clear that the use of data-uri’s for background-images is not a silver bullet, but if you have images that are on every page of your site and they’re small in file-size, migrating those into your CSS as background-images with data-uri’s can result in an important performance improvement for your site.

2 thoughts on “Dipping below the magical 1 second page load time

  1. Himanshu Jain


    I have been using your plugin since it’s launch and it is very useful to me. My recent tests are showing the combination of Autoptimize with WP Fastest Cache is resulting in better performance than with WP Super Cache. You should have a look once.

    Best regards,


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