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How important is my YSlow score?

As it turns out, your YSlow score is very important.

At Yottaa, we have been tracking the YSlow score of thousands of websites for a few months now and we thought it was about time to share some of our findings.

The average YSlow score is 69

Here?s a chart showing the distribution of YSlow scores over the 5000+ websites used in this sample.

Distribution of YSlow Scores

For those of you who are interested, the worst YSlow score we’ve tracked is for this site: A Site that is also interesting for many reasons besides its YSlow score.

[update: 9-13-2010. By popular demand, here’s a history of YSlow scores over time as tracked by Yottaa]

This chart shows the average YSlow score over time across all of the sites tracked by Yottaa. Our database of websites tripled in size from the left to the right side of this graph, so we think that the downward trend actually has more to do with the new websites added to the sample rather than an actual “un-optimization” of the web 😉 I’m working on an updated chart that tracks the same cohort of sites over time rather than the growing list.
Average YSlow Score Over Time

Performance gains from YSlow improvements decay logarithmically

Here?s a chart showing the average page load as it relates to the YSlow score of the site. We see that performance improvements drop off logarithmically as we get closer to that elusive ?perfect? YSlow score of 100.

I think that this chart is amazing. I didn’t expect such a perfect fit for the data.
Impact of YSlow score on page load time

It turns out that you?re going to need to add about 20 points to your Yslow score if you want to cut your page load time in half.

Improving YSlow leads to a more consistent user experience

Here you can see the standard deviation of page load time across a range of YSlow scores. The data points on the far left and far right are somewhat skewed due to the low number of sites that get scores at the extremes of the YSlow spectrum.

Standard deviation of page load time by YSlow score

Overall, we see a fairly consistent linear improvement in page load time deviation as we increase our yslow score. So even though you may get diminishing returns on an absolute speed performance by improving your score, the minimum and maximum load times will be more consistent as your score improves.

So if you’ve made it this far, what are you waiting for? Go check out your site’s performance right now!


The YSlow values used to compute these reports were generated by the YSlow plugin for Firefox version 2.0.1.

The Page Load values used to compute these reports were the onContentLoaded event as reported by the Firebug plugin for Firefox.

Yottaa provides a web performance monitoring service called “yottaa Insight” that collects the YSlow and Firebug samples 3 times per day from 4 different geographic locations (Singapore, California, Virginia, Dublin) The values used in these charts is the average of all of these measurements.

The websites used in this sample consist of approximately 5500 URLs entered by visitors to over the past 3 months.

Final note

We have tons of data about web performance. If you are interested in accessing or using this data, please get in touch with me.






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