Have you noticed that YouTube pages appear to load slower in Edge, Firefox, and Safari compared to Google's own Chrome browser? If so, you are not alone. Google redesigned the YouTube experience last year, but the site still uses an older shadow API that is only used in Chrome, which makes other browsers render YouTube much slower. Chris Peterson, a program manager at Mozilla, noticed the performance disparity and posted about the topic on Twitter. According to Peterson, the YouTube page loads five times slower in those other browsers "because YouTube's Polymer redesign relies on the deprecated Shadow DOM v0 API only implemented in Chrome."
YouTube page load is 5x slower in Firefox and Edge than in Chrome because YouTube's Polymer redesign relies on the deprecated Shadow DOM v0 API only implemented in Chrome. You can restore YouTube's faster pre-Polymer design with this Firefox extension: https://t.co/F5uEn3iMLR
That does not mean YouTube will not work in Edge, Firefox, and Safari-you can visit the site and load videos just fine using any of those browsers. However, it takes longer to render all those web elements compared to visiting YouTube from within Chrome. It's sort of a sneaky issue that could lead some users to think their Internet connection is simply slow. What is frustrating about the situation is that there are newer versions of Polymer available (2.0 and 3.0) that both support the deprecated API and would resolve the slowdown issue. As to why Google has chosen to stick with an older release is anyone's guess, and many are guessing that it is to push people over to Chrome.