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Stuff we've learned

How Javascript Execution Time Affects Bounce Rate Javascript Execution Time vs Bounce Rate

It’s no secret that Google likes web pages that load quickly. In fact they offer free page speed tools to help you optimise your site and make the bold claim that “Fast and optimized pages lead to higher visitor engagement, retention, and conversions.”

We’ve no reason to doubt them either – all of our previous testing bears out these claims. However, it recently became possible for us to measure the time it takes for javascript on our pages to finish executing, so we decided to have a look again, and while the overall result wasn’t surprising, the scale of it was.

Bounce Rate Increased 15%

In short, the difference between the bounce rate at 0 ms javascript execution technically (0 – 50 ms) and 1500 ms was 15%, or in our case a rise from 41.86% to 48.26%. More than that, the rise in bounce rate was almost linear as the javascript execution time got bigger. (After 1500 ms the bounce rate flattened out.)

The data above is taken from around 150,000 visits that started on our search results pages, the most viewed pages on our site. The result is similar for visits that start on our brochure pages, although the rise in bounce rate isn’t quite as large, peaking at 6.5% up between 100 ms and 900 ms javascript execution time.

What Next?

The data indicates a very direct correlation between the time our javascript is taking to execute and the bounce rate of visitors to our site, so next we’re going to dig in a little further to work out why the javascript takes such a range of times to execute, and see if we can work out any quick wins on how to shave some precious milliseconds off our current speeds.