Image optimisation improves page speed by 11%

If you impose a 100 kB limit on all the images on your website you can expect to improve page speed by an average of 11%. In some cases, it’ll be a lot more. This is a big enough performance increase to boost your search performance.

Image optimisation is one of those pain-in-the-backside tasks that we all know we ought to do but we never really want to. We have a sneaking suspicion that it could be a whole lot of work for very little return. Our tests show quite the opposite. It’s well worth our while.

We identified 20 pages on our website that contained images over the nominal 100 kB limit. We measured the speed of these pages using Google’s PageSpeed Insights, brought the images under the size limit and then retested the page speed. The results are in the following chart:

Image optimisation improved mobile page speed by 11% and desktop speed by 3% (averages).

Image optimisation improved mobile page speed by 11% and desktop speed by 3% (averages).

There are several points to note:

  1. The speed index for mobile devices benefits far more from smaller images than the desktop index. With Google having adopted a ‘mobile first’ approach, this is great news for any website.
  2. The biggest performance boost came from a change of image format, not just compression. We were advised to change an animated GIF image into a video.
  3. Although the average increase for mobile devices was 11%, there are plenty of pages where the increase was over 15%.
  4. The performance increases for desktop devices is far more modest (3% on average) but even this is significant. Search optimisation is a game of incremental improvements.
  5. Results will vary according to how many images you have on your pages and how well (or badly) optimised they are already.

The image optimisation process moved the average page speed for mobile devices from 68 to 73. The desktop average started at 92 so there was less scope for improvement. It ‘only’ went up to 95.

Image optimisation moved the average speed for mobile devices to 73 and desktop devices to 95.

Image optimisation moved the average speed for mobile devices to 73 and desktop devices to 95.


Image optimisation works for small, repeated images too

Although smaller images aren’t a problem per se, they can slow down page load times when they’re used repeatedly. Take a look at the thumbnails in this page’s Downloads boxes. They used to be 70kB each. We have numerous pages that offer four downloads. That’s suddenly 280kB. By reducing these to around 30kB we speeded up page load time without losing any valuable detail.

The beauty of changing repeating images is that improving one image boosts the performance of every page that uses it.


Is image optimisation worth it?

Anyone who thinks commercially rather than technically will still be wondering whether this is worth it.

We can tell you that it took about 15 minutes to optimise and replace each image. We can and have told you the performance increase you can expect to see from that investment in time.

But we can’t tell you (yet) how that will improve our search rankings. Nor can we say how much more traffic we’ll get from appearing higher on results pages. And nor do we know how much more business we’ll get from the extra traffic.


Image optimisation tools

You may question whether Google PageSpeed Insights is the best tool to measure the speed of a web page. To be honest, we don’t like it either. But it’s Google’s tool. If it’s Google’s search engine deciding if our webpages are worth showing, we’ll use Google’s tool to determine if they’re fast enough.

In case you’re wondering, we used Screaming Frog to scan our website and identify the images that were over 100 kB. Although there’s a paid version of the tool, the free version works on websites with less than 500 pages.