Simon Hearne

Web Performance Architect

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Four Charts That Prove the Value of Site Speed

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Site speed is critical to user experience and online success, here are four charts which prove the value!

❤️ 30 likes 🔁 6 reposts 💬 2 comments 🔗 3 shares as of 11:00, November 26


Site speed is an important but often-overlooked component in user experience. We know intuitively that slow experiences are unpleasant and make us more likely to leave a site, but building evidence of this effect has always been tricky.

The charts below were built with data collected using mPulse RUM.

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This chart shows the page speed distribution for an website by device type. The area is the distribution of sessions by average page speed and the lines in this case show the average bounce rate for sessions by the average page speed in the session.

The majority of traffic is mobile, hence the higher blue distribution. This website is well optimised for mobile browsers resulting in a faster average page speed than on tablet or desktop experiences. The distributions show a peak of session average page speed at around 1.2 seconds for mobile and desktop, with tablet lagging slightly at around 1.5s. Note that the page speed here is the average of all page views in a session: including the landing page, browsing and the checkout flow where appropriate.

Users are more likely to leave the site immediately when the first page of a session is slow.

The bounce rate lines here show that there is a strong correlation between page speed and bounce rate. The initial drop is due to error pages and bots which result in high bounce rates and very fast pages. There is a settling around the peak of the distribution where we find the optimal bounce rates of 22% on mobile, 14% on desktop and 9% on tablet, these occur at the fastest page speeds of around one second on average!

Users are more likely to leave the site immediately as their page speed gets worse. The mobile bounce rate increases from 22% to 40% as page speed increases from 1s to 2s! The median bounce rate for mobile is 41% at 2.7s, only 50% of sessions achieve this speed.

Your slowest users will not even appear in your data - they will have left before the page loads 😱

We see a similar pattern when looking at conversion rate. Conversion peaks where page speed is under a second, with conversion rate halving across all device types from experiences of one second to two seconds.

Users are less likely to convert if their average experience during a session is slow.

It's not just conversion that takes a hit when users have a slow experience. Average order value drops significantly in the first second here from $190 on desktop at 800ms to $140 at 3s. Faster experiences lead to bigger basket sizes!

Visitors spend more money when the average experience is faster.

If you make money through advertising, session length is a key performance indicator. In this graph we see that session length correlates with page speed too!

Users visit more pages when the average experience is faster.

Here the average session length peaks on desktop at 20 pages per session at a one second average page speed. This drops dramatically to just 13 pages when users have an average 2.25s page speed.

The charts presented here are generated from real data, the explicit values are examplar.

Conclusions

Page speed is a critical factor in user experience, but most visitors are not even aware of it! Slower experiences are reported to feel ‘basic’, ‘confusing’ and ‘clunky’, rather than slow. Delivering fast user experiences gives your website or application the best chance at meeting your business objectives and creating positive user experiences.


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  1. Michael Gooding

    I like the post but I actually think I am more impressed with the pinned graphs that change when I scroll on mobile. I missed it first time but actually it’s a great user experience!!

    reply
  2. Simon Hearne

    Thanks 😊 It’s more obvious on this post as the points move to let you know they have changed: simonhearne.com/2020/save-data…

    reply

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