Alternative hypothesis (assuming a bug rather than intended behavior): YouTube is testing a new ad mechanism, and the new mechanism (rather naturally) wasn't tested on systems blocking ads; it then produces a delay if the ad can't be downloaded, rather than silently skipping it. But yeah, I do tend to watch most of my videos through youtube-dl these days, even though the HTML5 player works fine for me in Firefox with…
So you also concluded: it's not a bug, but an intentional "feature" to suppress ad-blockers