Optimizing Website Speed with Preload, Prefetch, Preconnect, and DNS Prefetch

December 04, 2022

Preload, Prefetch, Preconnect, and DNS Prefetch are the methods that web browsers use to improve the performance of websites. These methods allow the browser to download or resolve the necessary assets for a page in advance so that the page loads faster when the user visits it. In this article, we will explore the differences between these techniques, and provide examples and best practices for using them to optimize the performance of a website.

Preload

The Preload resource hint is a declarative fetch instruction that tells the browser to download a specific resource as soon as possible, without blocking the rendering of the page. This is useful for resources that are essential to the initial rendering of the page, such as fonts, images, and other critical assets.

In this example, the browser will download the specified font file as soon as it encounters the <link> element, without waiting for the rest of the page to be parsed. This means that the font will be available to the browser immediately when it needs to render text using that font.

Prefetch

Prefetch is similar to preload, but it is used for resources that are not essential to the initial rendering of the page. Instead, prefetch hints are used to tell the browser to download resources that might be needed in the future, such as images on a subsequent page, or resources used by a different website. To use prefetch, you can include a <link> element in the <head> of their HTML, with a rel attribute set to prefetch, and a href attribute that specifies the URL of the resource to be prefetched. For example:

In this example, the browser will download the specified image file as soon as it has finished parsing the current page, but it will not block the rendering of the page. This means that the image will be available for use if the user navigates to a page that needs it, but it will not affect the performance of the current page.

Preconnect

Preconnect is a feature that allows the browser to establish a connection to a server in advance before a resource is actually needed. This can improve the performance of the page by reducing the time it takes for the browser to establish a connection to the server when the resource is needed. To use preconnect, you can include a <link> element in the <head> of their HTML, with a rel attribute set to preconnect, and a href attribute that specifies the URL of the server to be connected to. For example:

In this example, the browser will establish a connection to the specified server as soon as it encounters the <link> element, without waiting for the user to click on a link to that server. This means that the browser will be able to access the server more quickly when it needs to fetch a resource from that server.

DNS Prefetch

DNS Prefetch is a feature that allows the browser to resolve the DNS (Domain Name System) of a domain in advance before the user clicks on a link to that domain. This reduces the time it takes for the browser to establish a connection to the server, which can improve the overall performance of the page. To use DNS Prefetch, you can include a <link> element in the <head> of their HTML, with a rel attribute set to dns-prefetch, and a href attribute that specifies the domain to be prefetched. For example:

In this example, the browser will resolve the DNS for the specified domain as soon as it encounters the <link> element, without waiting for the user to click on a link to that domain. This means that the browser will be able to establish a connection to the server more quickly when the user does click on a link.

Difference between Preconnect and DNS Prefetch

The main difference between the preconnect and dns-prefetch is that preconnect establishes a connection to the server, while dns-prefetch resolves the domain's DNS. Preconnect can be more effective at improving the performance of the page since it allows the browser to establish a connection to the server more quickly when the resource is needed. However, DNS Prefetch is still useful for improving the overall performance of the page, by reducing the time it takes for the browser to establish a connection to the server.

Advantages

The advantages of using preload, prefetch, preconnect, and dns-prefetch include:

Improved page load times: By downloading or resolving the necessary assets for a page in advance, these methods can help to reduce the time it takes for the page to load, improving the user experience.
Reduced server load: By downloading assets in advance, these methods can help to reduce the number of requests that the server has to handle, which can improve its performance and reduce its load.
Enhanced user experience: Faster page load times and improved server performance can lead to a better overall user experience, with pages that load quickly and smoothly.
Improved resource availability: By downloading resources in advance, the prefetch resource hint can help to ensure that the necessary assets are available when needed, allowing the page to render correctly and quickly.
Improved prioritization of resources: By explicitly specifying which resources should be downloaded first, the preload resource hint can help the browser to prioritize the loading of critical assets, ensuring that the page renders correctly and quickly.

Disadvantages

While preload, prefetch, preconnect, and dns-prefetch can be useful for improving the performance of a website, but there are also some potential disadvantages to using these methods. Some of the cons include:

Increased bandwidth usage: These methods can cause the browser to download or resolve more assets than would otherwise be necessary. This can increase the amount of bandwidth used by the page, which can be a problem for users with limited or expensive data plans.
Unpredictable behavior: The behavior of preload, prefetch, preconnect, and dns-prefetch can be difficult to predict, especially when multiple resource hints are used on a single page. This can make it difficult for you to control the behavior of the page, and to ensure that the resources are being downloaded or resolved in the most efficient way.
Ineffective on slow connections: These methods are most effective on fast connections, where the assets can be downloaded or resolved quickly. On slow connections, these methods may not provide much benefit, and can even reduce the performance of the page.
Reduced control over caching: These methods can cause the browser to download or resolve assets without considering their caching status. This can lead to unnecessary downloads and can reduce the effectiveness of caching mechanisms.
Potentially confusing to users: The use of preload, prefetch, preconnect, and dns-prefetch can be transparent to the user, which can be confusing or even misleading. For example, if a resource is preloaded but never actually used on the page, the user may not understand why their bandwidth was used for that resource.

Tips and Tricks

Here are some tips and tricks for using preload, prefetch, preconnect, and dns-prefetch to improve the performance of a website:

Use preload for critical assets: Preload is best used for resources that are essential to the initial rendering of the page, such as fonts, images, and other critical assets. This will ensure that the assets are available to the browser as soon as they are needed, improving the performance of the page.
Use prefetch for non-critical assets: Prefetch is useful for resources that are not essential to the initial rendering of the page, such as images on a subsequent page, or resources used by a different website. This will allow the browser to download these assets in advance, without blocking the rendering of the current page.
Use preconnect for frequent domains: Preconnect is most effective when used for domains that are accessed frequently, such as CDN servers or third-party APIs. This will ensure that the browser establishes a connection to these servers as soon as possible, improving the performance of the page.
Use dns-prefetch for external domains: DNS Prefetch is useful for resolving the DNS of external domains that are linked to from the current page. This will allow the browser to establish a connection to these domains more quickly when the user clicks on a link, improving the overall performance of the page.
Use the appropriate as attribute: When using the preload, it is important to specify the
as attribute, which tells the browser what type of resource is being preloaded. This will allow the browser to prioritize the loading of the resource, and to handle it correctly when it is needed.
Avoid overusing resource hints: While these methods can be useful for improving the performance of a website, it is important not to overuse these methods. Too many resource hints can cause the browser to download or resolve unnecessary assets, which can actually reduce the performance of the page.
Test and measure the impact: To determine the effectiveness of preload, prefetch, preconnect, and dns-prefetch, it is important to test and measure the impact of these methods on the performance of the website. This will allow developers to optimize their use of resource hints, and ensure that these methods are providing the maximum benefit.

Conclusion

Overall, using preload, prefetch, preconnect, and dns-prefetch can help to improve the performance of a website, making it more responsive and user-friendly. But it is also important not to overuse these methods because that can cause the browser to download or resolve unnecessary assets, which can actually reduce the performance of the website.