Optimizely Blog—Page Load Time & Engagement

Oliver Palmer wrote a post on the Optimizely blog about how page load time impacts engagement. Recognizing the increasing prevalence of third-party javascript…

between the homepages of 20 major US and European publishers, some 500 different external snippets of Javascript were loaded. Nine months later, that figure has risen to almost 700.

…they set up an A/B test experiment with The Telegraph.

Do these hundreds of lines of externally loaded Javascript code impact how fast a page loads? By how much? And what impact does this have on user engagement and retention?

We would artificially slow the site down in order to measure the impact on overall user engagement and retention to try and model out the relationship between site speed and overall revenue.

Delays ranged from 4 to 20 seconds, with page view impact ranging from -11% to -44%. That’s a significant drop-off, but not particularly surprising given similar documented cases.

Using a metric developed by the Telegraph’s internal strategy team representing the monetary value of a pageview[…], we were able to model out the overall revenue impact of each variant. By doing so, we could paint an accurate picture of the cost to user engagement of any new on-site changes which incur a detrimental impact on page performance.

This last step seems crucial. Establishing a monetary value for a page view can help facilitate objective discussions around tradeoffs between revenue gains and impact to user experience. I’d love to learn more about how/if this new data led The Telegraph to many any changes.