Measuring the benefits of a CDN


A few months ago we integrated Rackspace's Cloud content delivery network (previously branded as Mosso CloudFiles) into our ecommerce product, salesonrails. We've now completed a similar integration into our content management system, fabric.

A content delivery network (CDN) distributes copies of files to various servers around the globe, allowing users visiting a website to download content from a server geographically closer to them. When used for small files embedded in a page (e.g. template images), the CDN should result in improved page loading time. When used for larger files that a user might click on to download (e.g. downloadable PDF documents), the user should see faster download speeds.

Tools such as YSlow often recommend using a CDN to improve performance, however since the most benefit will be seen by international visitors to your site, it can be hard to see improvements first hand. Thankfully there are some free online tools to help. Our favourite of these is, which simulates a browser viewing your site, times the download of individual elements on the page, and gives you detailed reports.

We setup a test page on one of our UK hosted fabric sites, consisting of 200kb of total content (page weight, CSS and some large images). We then ran this page through the testing tool, both with and without the CDN enabled. Here are the results (all for the 'first view' - simulating a new visitor to the site who won't have anything cached):

Simulating a visitor from the east coast of the US (lower times are better):

Load timeTime to first byteStart renderDocument completeFully loaded
Without CDN3.252s0.421s1.226s3.252s3.252s
With CDN2.751s0.421s1.104s2.751s2.900s

This shows a 0.5 second improvement in the total load time for the page.

Running the test again, this time using the New Zealand location showed even more of an increase:

Load timeTime to first byteStart renderDocument completeFully loaded
Without CDN5.519s1.061s2.119s5.519s5.519s
With CDN3.823s1.061s1.997s3.823s4.210s

so with the CDN on, the same page content loaded almost two seconds quicker.

This shows that even though the benefits of a CDN may not be immediately obvious on sites you host locally, your international visitors are going to see noticeable improvements.

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