Prefetching refers to the downloading of contents based on the action that a user is most likely to make next. The idea behind it is to offer access to resources faster if the user happens to access one of the preloaded contents in the browser.
One primary example is Google Search where the majority of users click on one of the first results. If a browser would preload these resources, they could be accessed faster and benefit most users as they do access these resources.
The disadvantage of prefetching contents is that you are wasting bandwidth if another resource is accessed or none at all. Prefetching can be disabled in Google Chrome.
Google introduced the concept of reactive prefetching recently in Google Chrome for Android. Unlike original prefetching, it is starting to load specific contents after the user action.
This is done through the use of hints on the source site. Reactive prefetch is currently only working on Google Search and only if Google Chrome for Android is used to access the page.
The resources that the source page hints at are downloaded in parallel with the destination page which is the main reason why pages are loaded faster than before in Chrome for Android.
Chrome saves between 100-150 milliseconds to render web pages if reactive prefetch information are provided on the source page.
According to Ilya Grigorik, Google is currently working on bringing reactive prefetch to the desktop versions of Google Chrome.
100-150 milliseconds may not seem like much and it is likely that many users won't notice the difference in page loading time if you take reactive prefetch on its own.
Good news is that it won't waste any bandwidth as it only prefetches contents that are loaded anyway in the web browser.
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