This is reported by Android Authoruty.

It turns out that this is a new YouTube tactic that aims to encourage viewers to subscribe to YouTube Premium, which provides access to ad-free content. This was reported by the YouTube press service.

“We have tried to encourage viewers who have ad blocking enabled to allow ads or try Premium to use the service without ads,” YouTube’s press service said.

However, this does not explain why the five-second delay disappears when using Google Chrome, even when using blockers.