edit4: other talked about video on subject edit3: added andriodauthority paragraph edit2: people mention only some experience this, may be adblock testing on yt part, code below may not be cause edit: added code and andriodauthority link, also in desc. of video

He talks about an artificial 5 second timeout function: Polymer script that has 5E3 (5000ms) in script

https://www.androidauthority.com/youtube-reportedly-slowing-down-videos-firefox-3387206/

https://cybernews.com/tech/firefox-users-frustrated-over-alleged-delay-on-youtube/

setTimeout(function() {
    c();
    a.resolve(1)
 }, 5E3);

Certain discussions around the report indicate that the code could be a lazy implementation of an ad fallback if a user uses an ad blocker. The relevant code could possibly be ensuring that an ad is displayed for at least five seconds before the actual video begins showing. As mentioned, we could not confirm the functioning of the code snippet.

YouTube Has Gone Too Far This Time [10:49 | Mental Outlaw] https://www.youtube.com/watch?v=v4gXhmzQztE