Not trying to defend Chrome here as I dislike their other behaviours, but just from what's presented in the video, an alternative explanation would be caching. That is, when the reloading is triggered by the switch of user-agent, the cache is reused and thus a shorter load time.

To exclude this effect, the user needs to either

1. Spoof the user-agent and at the same time clear cache (you can disable cache when reloading through the developer's tool), or
2. Clear cache, spoof the user-agent to Chrome. Load page, disable the spoofing, reload.

If this is true the crowd on here that often says Firefox is really owned by Google because Google pays Mozilla to have their search engine be the default search engine on Firefox really need to look at their claim and rethink their understanding of how Mozilla and Google interact.

I can confirm this. I signed up for the free trial of YT Premium (which I immediately cancelled, but I'll take the three months) but YT seems to be detecting that I'm on Firefox and have Ublock installed, so I'm getting the occasional 5 second forced delay, even though I'm a premium user. (They may not count the trial as being a "true" premium user, I'd suspect.)

Well that sounds illegal, it also sounds like Mozilla will see them in court.

I’m seeing this on Safari with the AdGuard content blocker extension as well.