I/we have a few Wordpress based sites that generally sustain 40,000 to 60,000 uniques per day, with occasional peaks to much higher per day. They are not tubes, but do include some videos.
WP can be fairly efficient once you employ caching, a reasonably powerful dedicated server, sufficient RAM, and reduce/eliminate 3rd party plugins as much as possible. But out of the box and using a bunch of plugins, I don't think WP would be exactly efficient on higher traffic sites. Certainly it needs to be tweaked and the hardware/technical setup sufficient to scale to the traffic load.
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