Since YouTube's redesign launched last year, users of the video streaming service that don't use Google Chrome have experienced significantly slower page loading when attempting to watch videos. There are methods to revert to the site's classic layout, which resolves this issue for the time being.
As explained by Chris Peterson, technical program manager at Mozilla, on Twitter, the difference between YouTube's loading times on Google Chrome versus its competitors Mozilla Firefox and Microsoft Edge is five-fold. This is due to the fact that the new YouTube design utilizes a deprecated Shadow Document Object Model application programming interface. This deprecated Shadow DOM API is only supported by Google Chrome.
The steps to speed this up by restoring the old YouTube layout vary based on which web browser is being used. In order to restore YouTube to its pre-Polymer redesign, Firefox users simply need to install the YouTube Classic Firefox add-on. Edge or Safari users, however, must install a userscript manager (such as Tampermonkey) in order to install Cpt_mathix's Youtube - Restore Classic userscript.
Those that decide to revert to YouTube's classic layout in order to speed up their video watching will have to sacrifice the newly introduced features to the Polymer-based redesign, such as built-in dark mode support.