A Content Delivery Network, often referred to by the acronym "CDN", is literally a "content distribution system". It relies on the use of servers installed in datacenters located at strategic points around the world. These are interconnected via very high-speed private fiber optic connections, allowing web content to be delivered more quickly to users.
The concept of a CDN is therefore intimately linked to page loading speed. However, its use does not address all web performance issues, and is not a systematic solution to web performance problems. In this article, we will explore together what activating a CDN can bring in terms of performance and security, as well as the side effects it can cause.
Advantages of using a CDN
Cutting-edge network technologies
Since CDNs are designed by network experts like Akamai, Cloudflare, Amazon CloudFront, or Fastly, they benefit from cutting-edge, highly available infrastructures. They are better equipped than 90% of traditional web hosts. This allows for the use of technologies that are still not widely deployed, such as the HTTP/3 protocol, Brotli compression, or Early Hints.
It is possible to take advantage of these technologies on a classic server, but at generally much higher costs: this requires, in particular, having a powerful virtual private server (VPS) or a dedicated server. And entrusting its management to an experienced system administrator, which also has a cost.
Faster DNS resolutions
CDNs function as a proxy, meaning they interface between the origin server (your hosting) and the end-user (your visitor). To do this, they are entrusted with DNS management, usually handled by the registrar. However, CDNs have extremely high-performance DNS infrastructures, which translates into lower response times (Time To First Byte, TTFB metric).
Cloudflare, which is exemplary in this regard, thus shows DNS resolution times 3 times faster than OVH, the leader in web hosting in France. Note also that it is entirely possible to entrust DNS management to Cloudflare without activating the CDN functionality, and this for free. You will then benefit from the best available at this level, with an interesting impact on TTFB.
Fast pages all over the world
This is the primary use case for a CDN, the one and only that it can fulfill! Thanks to the ultra-fast network connections linking all of its data centers, a CDN can serve a page to any user in the world in a matter of seconds. If you have hosting in France and a significant portion of your traffic comes from abroad, then the question of a CDN must be considered.
When choosing your CDN provider, make sure that it has data centers in the regions where your visitors are located. Because while Cloudflare, Akamai, and AWS all have perfectly meshed infrastructures, other smaller providers only offer a few dozen locations. Response times will not be as good in this case since the data will have to travel long distances.
Optimized static content
One of the basic mechanisms of a CDN is the copying of static files to its own servers: upon the first access to an image, video, stylesheet, font, or JavaScript file, the resource is cached on the fly and transparently... When a user accesses a website, the CDN automatically redirects requests to the data center closest to the user, significantly reducing loading times.
Most CDNs, however, offer to go further by applying on-the-fly compression, minification, or even conversion of certain resource formats to others that are lighter. The most common example is generating images in Webp or Avif as an alternative to traditional formats like PNG, Jpeg, and Gif. This considerably reduces their size while maintaining equivalent or slightly degraded image quality (as chosen).
This second point can be managed directly by hosting, but with the need to deploy specific and often paid tools. This can also impact disk space requirements: multiplying the volume occupied by images by 2 or 3 may require upgrading to a hosting plan with more storage space.
Lower hosting costs
By absorbing a majority of HTTP requests, a CDN greatly relieves the origin server. The latter is generally only responsible for generating dynamic pages, with everything else being served directly by the data center closest to the visitor. This results in a significant decrease in CPU and RAM resource consumption.
It is up to you to keep this reserve of available resources, to absorb potential one-off traffic spikes, for example. Or to downgrade your hosting by switching to a less powerful, and therefore less expensive, offer. In this second case, however, be careful not to underestimate the need: if the CDN cache is emptied, all subsequent requests will be directed to your hosting, which must be able to handle them.
Finally, do not forget that CDNs are also paid: to see if the solution allows for savings, you should compare the cost of the initial hosting vs. the new hosting + the CDN.
Increased security
Since CDNs function as proxies, they offer significant security advantages. To begin with, your server is no longer directly exposed to end-users: they cannot know its IP address. It is impossible for hackers to attempt intrusions via FTP, SSH, or web without going through the CDN provider's firewall. And these providers are, once again, at the forefront of what is available.
CDNs also integrate advanced protection against denial-of-service attacks (the famous DDoS, for "Distributed Denial of Service"). They can instantly and transparently switch to an intelligent mitigation interface. And, once the crisis has passed, return to normal operation without you even realizing the danger that was avoided.
Is your site as fast as your visitors expect?
Disadvantages of using a CDN
High technical complexity for novices
Deploying a CDN can be complex for a website publisher accustomed to all-in-one services from their web host like OVH, Ionos, or Gandi. This involves changing the DNS servers, a step that is often a deterrent for non-technical users. Subsequently, it is important to verify that the DNS records have been correctly re-entered in the new interface. Then, wait for DNS propagation to occur, which sometimes takes time.
Once the DNS is managed by the CDN, the proposed options must be configured appropriately. By default, most additional optimizations are disabled. Therefore, it is up to the website publisher to configure the features optimally... which is often not done. As a result, part of the performance gain potential is lost, which is regrettable.
A potential risk for crawling
Gary Illyes, a well-known analyst at Google, published a message on LinkedIn explaining that one of the most common crawling issues was related to the use of a poorly configured CDN and/or Firewall. The sometimes aggressive security settings of these tools can block search engine indexing robots, preventing Googlebot, among other things, from indexing a site's pages.
The recommendation is to systematically check the CDN configuration and add the IP address ranges used by known indexing robots to its whitelist. If a site is inadvertently blacklisted, the effects on SEO visibility can be catastrophic.
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Most CDN providers offer a basic service, which mainly includes distributing content across various datacenters with an added security layer. Depending on the size and traffic of the site, this can already represent a significant cost compared to renting a server from a web host.
Other advanced options such as configuring Firewall rules, enabling Brotli compression, or generating Webp and Avif alternatives may incur additional costs. If you rely on a CDN to optimize your site's performance, be sure to compare what different providers include in their basic offering and project costs for variable-priced options: they often depend on the volume of bandwidth consumed.
Finally, before making any decisions, don't forget to consider renting a more powerful server configured by an experienced system administrator: the cost will likely be significant here too, but the variability factor will disappear in favor of a fixed cost that is easier to budget.
An additional single point of failure
Although they have highly available infrastructures, CDNs have all the technical characteristics of what is called a Single Point Of Failure (SPOF). More concretely, if your CDN experiences a problem or technical failure, your site will no longer be accessible or may be significantly slowed down.
These scenarios are rare but have already affected major players like Cloudflare, which, on June 21, 2022, for example, faced a major network outage. Hundreds of thousands of websites were unavailable worldwide for about 2 hours, until network technicians could detect the cause of the outage and fix it.
Perverse effect for reduced geographic targeting
A CDN primarily serves to improve loading times for users who are geographically distant from the origin server. If the majority of your users are in the same geographic region as this server, using a CDN is probably not justified.
Worse still, implementing a CDN will very often have a negative impact on page response times if users are close to the origin server. Unlike static resources, dynamic pages generally cannot be cached by CDNs. With each request, the CDN will fetch the page from its source, then offer it to the visitor in turn.
This additional step can add several tens or even hundreds of milliseconds to the TTFB, consequently impacting other performance metrics such as First Contentful Paint (FCP) and Largest Contentful Paint (LCP). If you have hosting in France and 95% of your visitors live in mainland France, a CDN is not the magic bullet for your web performance issues.
So, CDN or not CDN?
At the decision-making stage, it is important to be pragmatic. If I had to summarize my opinion on CDNs given the advantages and disadvantages presented, here it is in a few key points:
- It is an optimal solution for globally frequented sites, but not for highly geolocated sites
- It is not the magic bullet for performance issues: you can also optimize the site on its existing hosting
- It provides an undeniable security benefit
- It is not necessarily less expensive than high-performance hosting
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