I noticed this in Opera stable (version 20) but it could potentially be the same for Chrome and Firefox.
Simply put, Flash video playback is terrible. As much as I'd like to switch to the HTML5 player in YouTube, there's a current active issue between YouTube and Opera that is blocking the HTML5 player, so the browser insists on using the Flash player.
Similar thread here: http://forums.adobe....message/6305593
I can't reproduce this in IE11, since over there it automatically uses HTML5 which plays beautifully well.