What should I know about GA Site speed reporting details?

I use Google Analytics for all my website data reporting. Is the site speed data accurate?

Google Analytics (GA) has become the defacto data source for information related to your website.

GA is an incredibly powerful tool, the data that is gathered for everything except site speed is a reliable, actionable source of information.

In its default configuration, it’s impossible to get a reliable site performance figure from GA. This is due to several reasons:

  1. Google Analytics sample rate for speed related data is incredibly low

This is the biggest issue with the product as by default only 1% of your traffic is sampled. This means that the lion’s share of your user activity is not tracked and reported.

Depending on traffic volumes this often means that a couple of slow sessions that are picked up by GA will throw all speed data out.

Here is an example:

This site has recorded 513,053 page views in a month period. During this time the speed data is based on the performance of 2,519 pages.

Site speed data is based on 0.49% of the actual traffic.

Site overview:

Site speed overview:

It is possible to fix this issue to help improve the reliability of reports.

Ref: Google Developers Page

  1. Google Analytics measures the Mean (Average) of your speed data

The metric that is most important to site performance is Median. This is the point that is the middle of your users’ site performance and the best measure of actual user experience.

Mean is a common metric which is widely understood but it is susceptible to being skewed by outliers. Without a very big data set (volume of traffic) the mean figure will be thrown out by a handful of large outliers (ie. several slow page loads can throw all of the reporting data out even though the majority of your users are having a good experience).

This is amplified by issue 1 discussed above as it reduces the dataset size to < 1% of what it should be.