I couldn't reproduce the 5s wait in multiple scenarios in Firefox (various combinations of being logged in / not being logged in / without adblocker / with adblocker) and couldn't reproduce a 5s wait time in any of them, it played back immediately in each case (when without adblocker, using a second video to have one start without ad). I tested on Linux.
What exact combination of circumstances is required to trigger the multi second wait time?
I just tested this in firefox on ubuntu. Three subsequent new tab tests.
Load: 4.34s, 5.14, 2.96, 3.35
DOMContentLoaded: 3.65s, 4.56, 2.92, 3.33
Finish: 13.14s, 10.77, 8.49, 12.02
So it's getting a bit faster over time, but still heinous, and crucially, it isn't hanging on requests. Individual asset GET/POST requests are taking tens of ms, worst was a few parallel 254ms GETs on a cold start. Usually 50-70ms. But there is a flurry of requests, then a period of very few requests until 5s after init, then another flurry.
Same OS, chrome 115.0.5790.170, no blockers, youtube is much snappier, it still definitely takes a few seconds to paint thumbnails, but it's basically done by 5s. DOMContentloaded is never more than 1.75s, finish <8s.
Firefox private window with blockers off has similar time statistics. But actually doubleclick.net is still getting bounced.
I tested in Firefox (uBlock), LibreWolf (uBlock), Safari (AdGuard), and Chromium (no ad blocker), and the initial home page load takes a couple seconds, but I never witnessed a 5s delay. I would say it was actually fastest in Firefox for me, but that may have just been a result of some caching. I am a premium subscriber and have never seen a warning for using an ad blocker, so I'm not sure if premium subscribers get a pass.
What exact combination of circumstances is required to trigger the multi second wait time?