This is a deliberate deterioration in viewing
The YouTube video platform team has responded to a scandal flaring up online over complaints from users of third-party browsers about slow loading of videos.
Users have complained that lags occur when playing YouTube videos in browsers other than Chrome, such as Firefox, Vivaldi and Opera. However, a YouTube spokesperson released a statement saying that there is indeed a delay, but it has nothing to do with browsers.
Judging by the statement, this is not a glitch, but a deliberate measure of influence on those YouTube viewers who continue to use ad blockers.
The YouTube post says:
To support the diverse ecosystem of creators around the world and provide billions of users with access to the content they love on YouTube, we're trying to encourage viewers who have ad blocking enabled to allow ads on YouTube or try YouTube Premium for an ad-free experience. Users who have ad blockers installed may experience a suboptimal browsing experience, regardless of which browser they use.