What is Prefetching?

Prefetching is a performance optimization tactic in which content that might be accessed by the user is downloaded in advance. The browser (Chrome, Firefox, etc.) caches this content in the background, making it instantly available if the user clicks on a link that uses the content.

Unlike the static, text-based websites of the early Internet, today’s websites are driven by dynamic content and multimedia. A single web page can require multiple images and videos to be cached, resulting in long wait times and a poor user experience.

Prefetching resolves this issue by assuming which links the user is likely to click, then downloading the content of those links. If the user decides to click on one of the links, then the page will be rendered instantly as the content has already been downloaded.

This entry primarily discusses link prefetching, but there are other types of prefetching that are frequently performed.

DNS Prefetching

A typical domain lookup can take anywhere from 1ms to several seconds depending on whether the domain name is cached locally. DNS prefetching reduces domain lookup times by resolving the domain name in advance.

For example, Google Chrome implements DNS prefetching by scanning the hyperlinks in a page, extracting the domain names, and resolving them to IP addresses while the user browses the page. DNS prefetching is best used when there are multiple links that refer to external websites, such as the results in a search engine.

Prerendering

Prerendering is an extension of prefetching used by Chrome and Internet Explorer.

While prefetching downloads the content of a webpage, prerendering renders the entire page in the background. The prerendered page is treated as a separate tab, so when the user clicks on a prerendered link the current tab is essentially replaced by the prerendered tab.

In some cases, Chrome and Internet Explorer can analyze the content and layout of a website in order to perform automatic prerendering. For example, online periodicals can use prerendering to instantly serve the next article when the user is finished with the current article.

Internet Explorer 11 introduced Page Prediction, which analyzes browsing habits and page content to perform automatic prefetching. When a user opens the page, Page Prediction scans for links that the user is likely to click on, such as page numbers in an article. The browser downloads the resources for those links in the background, letting the page load almost instantly when the user clicks on it.

Most modern web browsers also allow resources to be individually prefetched using a “prefetch” tag. This tag identifies specific resources that should be prefetched in addition to the browser’s normal prefetching behavior. The prefetch tag allows web developers to overcome cases where the browser can’t predict which resources will be used.

Internet Explorer’s extended prefetching features include caching DNS queries, limiting prefetching for metered connections and disabling prefetching to conserve battery life. Browsers will always prioritize active downloads over prefetching and even pause prefetching if bandwidth is limited.

Prefetching ultimately results in a more fluid end user experience.

• Users experience faster load times for web content as pages are downloaded in the background. There’s no interruption to the user’s current browsing experience.
• Administrators see less spiked bandwidth usage since the content the user is likely to see has already been downloaded. Resources are delivered as needed, rather than all at once.
• Enterprises see higher visitor retainment as visitors no longer need to wait to access a web page. The browsing experience feels like the website is being hosted locally.

The fastest way to lose readers and customers is to make them wait for your website to load. Prefetching gets a one-up on latency by downloading web content before the user requests it. Users are left with a snappy, responsive browsing experience that feels like it’s running completely off their computer or mobile device.

As the average global Internet speed increases, so will users’ expectations. As the average global connection speed approaches 4.6 Mbps, delays could spell disaster for large websites. With a bit of behavioral analysis and developer creativity, prefetching works to make the web faster.

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