I'm having a hard time making heads or tails of the Site Speed graphs in Google Analytics. Our site speed is fluctuating wildly from month to month, despite a large sample (the report is "based on 100,000's of visits) and a consistent web set up (static files served from an EC2 instance running nginx behind a load balancer).

Here's our site speed, with each datapoint representing a week worth of data. Over this time period we modified our source and HTTP headers to increase our cache hits on static resources by 5x.

Why would it fluctuate so much? Is there any way to get more reliable information from those graphs?

1 Answer 1


There are so many factors at play it's really tricky to suggest without knowing a lot more about your site and the way it's used.

If you're looking at a large site I would suggest breaking down the data by location, it's likely that one geographic region is skewing the results. Also look for pages which contain very large images, or large numbers of images but are infrequently accessed.

GA is prone to results being skewed by a small number of outlying data points, especially in custom reporting.

Caching will only get you so far, you should consider a CDN, read Yahoo!'s Best Practices for Speeding Up Your Web Site and look at page level optimisation in addition to site wide optimisation. i.e. if you're loading lots of scripts, some may only be needed on certain pages. CSS sprites can also offer tremendous benefits to medium and high traffic sites.

See Google support's Interpret Site Speed (my emphasis):-

In speed analysis, looking at the average doesn't always tell the whole story, because a few outliers can skew the average. That’s why we recommend you look at the Performance tab to understand the distribution of your load/execution times.

And About Site Speed (my emphasis):-

On the Page Timings report, the Explorer and Map Overlay tabs contain two sections each: the Site Usage section and the Technical section. Site Usage shows your speed data at a high level in context of other Analytics metrics. The Technical section shows your data in more detail, providing a breakdown of network and server time metrics. The six sections on the Performance tab display different metrics grouped by speed ranges.

  • 2
    Good answer. Another tip if you are using Page Timings to see which pages are slowest: click "advanced" and add a filter for "Page Load Sample" greater than 30 or 50 to make it more reliable. Jun 20, 2012 at 9:06

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.