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YouTube has found a new way to load ads | AdGuard Blog
(adguard.com)
This is a most excellent place for technology news and articles.
It has to know which blocks to chose to get the next part of the file anyway. Except the next part of the file is an ad. So yes there is overhead but not for the video stream server. It doesn't need to re encode the video. It's not any more taxing than adding the non skip ads at the beginning that they already do.