What does Content Delivery Networks (CDNs) mean in SEO?

Content Delivery Networks are services offered by companies which can help you speed up page loading times (an important ranking factor) by storing parts of your website on multiple servers around the globe. This way, no matter where your visitors are located, some files that are needed to show pages on your site are delivered form local services.

Here's an example:

Let's say I have a shop selling glassware worldwide and the server that hosts my website is located in New York. Someone located in Perth (Australia) who opens a page on my site will have to wait while the text, images and other files needed to make my page display properly are transferred from New York to Perth.

The delay caused by sending files thousands of miles might only be 1 second but this can make all the difference on where you rank, especially in competitive markets.

I can use a CDN to keep my images (often the largest files that take the longest to travel) on servers all around the world including in or close to Perth. Now when my potential customer wants to open my page all the stuff which needs to be constantly up to date (the description, price, etc) will be sent from New York but texts are tiny files that move quickly.

The images (and perhaps things like Javascript files which control interactive elements on my page) are delivered to my visitor from a server just around the corner from them.

CDNs cost!

Content Delivery Networks are run by private companies maintaining multiple servers around the world. That's not cheap.

Some (like Cloudflare) offer a limited free package but for websites with any real traffic worth talking about you're going to have to get on one of their paid services.

Who needs a CDN?

Content Delivery Networks are ideal if you want to attract a worldwide audience. If your focused on one particular country or geographic area and your hosting company is located in that area (and they are a good quality host) you won't see any real benefit.

Don't use CDNs to make up for hosting on poor quality low speed servers as there will be elements that need to come from your host and if they aren't responsive your rankings will suffer.

I use Content Delivery Networks now and then but if you actually sort out a good host and carry out all the page loading speed setups explained here you might find CDNs don't really improve your rankings ... even for audiences located far from the server your site is on.