YouTube launched a design refresh last year, and one of the changes that Google implemented on the video streaming site was to make use of a deprecated API that only Chrome supported. Subsequent page navigation perf is comparable. On my laptop, initial page load takes 5 seconds with the polyfill vs 1 without. YouTube serves a Shadow DOM polyfill to Firefox and Edge that is, unsurprisingly, slower than Chrome's native implementation. 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. Peterson stated the following on Twitter: A Twitter message by Mozilla's Technical Program Manager Chris Peterson has created heated discussions on Twitter, Reddit, and elsewhere about YouTube's use of a deprecated API that favors Chrome and other Chromium-based browsers over competing browsers such as Microsoft Edge and Firefox.