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IRUK Top500 Speed Index: Measuring Top100 Website Performance

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Customers hate delays in being served. This retail adage has become clearer and clearer in the digital age, in great part thanks to usability experts showing how even incremental improvements in page speed times can have a significant impact on a retailer’s margins and profitability.

“Online shoppers hate waiting and there’s plenty of evidence that slower pages lead to lower conversion rates, higher bounce rates and an overall decrease in visitor engagement,” says Andy Davies, associate director for web performance with InternetRetailing Knowledge Partner NCC Group and an expert on-site optimisation. This is as true in the mobile sphere as it is with desktop.

A recent study by DoubleClick found that “53% of mobile site visits are abandoned if pages take longer than 3s to load”. Despite this, many mobile sites don’t achieve this. “The average load time for mobile sites is 19s over 3G connections,” DoubleClick noted when revealing the results of the study. “That’s about as long as it takes to sing the entire ‘Alphabet Song’!”

For busy retailers, it may seem that shoppers are being unreasonable in their expectations as they bail out of the ‘Alphabet Song’ somewhere around ‘d’ or ‘e’, but it’s important to realise we often can’t help ourselves but to click away.

“It’s not that shoppers are being impatient,” says Davies, “it’s just that subconsciously people are aware of even sub-second waits, and a delay of just a few seconds increases our stress levels and forces us to concentrate harder.” In itself, this explains why the idea of a Speed Index (see description below), which, in Davies’ words, measures “how long the visible part of the page takes to complete” is so powerful. While it may initially seem an overly abstract notion, the Speed Index we’ve used in our research is a useful tool for web optimisation.

Speed Index Explained

Speed Index is a measure of how quickly the visible parts of a web page are displayed. It is worked out by looking at the video frames of a page as it renders. Each frame is given a score for visual incompleteness above the fold. So the score is 100% for a blank screen and 0% for a visually complete page. The next step is to multiply that score by the number of milliseconds since the last frame (or since the beginning of the test if it’s the first frame). The final step to working out the Speed Index is to add these numbers together. The total is the Speed Index score. Lower scores indicate faster loads but the number is abstract rather than a direct measure of speed. For more technical detail on the metric, read

This post is an extract from the upcoming IREU 500 Mobile and Cross-channel Report, which will be released with the November issue of InternetRetailing Magazine. Subscribe here.

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