Why opening YouTube in Firefox or Edge can take 5 times longer than Google Chrome

Chris Peterson from Mozilla shared an unexpected discovery today with the public : as it turned out, at the moment, opening the YouTube page in Firefox and Edge browsers is 5 times slower than in Google Chrome. What is the reason for this phenomenon?

The fact is that the new YouTube redesign, which is based on the Polymer library , relies on the Shadow DOM v0 API, which was implemented exclusively in Chrome , and since April of this year it has been completely deprecated .

Because of the reason described, for updating YouTube to work in Firefox and Edge, a polyfill is used instead of Shadow DOM v0 - which, of course, works slower. On Chris's laptop, the initial loading of the page takes 1 second in Chrome, and under Firefox using the polyfill it takes as much as 5 seconds. During the subsequent navigation through the site, there was no performance drop. Under IE11, YouTube still works in the old design without using Polymer.
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The reason for this decision is unknown. Perhaps Google’s internal metrics have shown that a redesign with a polyfill increases user engagement in Firefox and Edge more than the slow loading of the site hurts them.

Most likely, the problem will go away only after a potential update of the YouTube redesign to Polymer 2.0, which is built on the Shadow DOM v1 API. But due to the fact that the implementation of the Shadow DOM v1 API in Firefox and Edge is still in development (the new version of the standard is currently supported by Safari, Chrome and Opera) and will be included only in the upcoming release of Firefox v63, now everything is resolved Anyone can return the old YouTube design with the YouTube Classic extension. What Edge users do - it is not specified to use the Tampermonkey extension and YouTube Restore Classic .