This is a long process on how to download from Youtube but for me, I found it with the most speed. You can use VideoDownloadHelper, the Firefox add-on but it will make a lot of problems if you want to download fast. In my regular speed with my broadband, I am supposed to get 350kb of speed. But when I try to download with Downloadhelper, it gives me the speed of 80-85kbps. So I tried to use the addon Downthemall.
But there is also a problem with Downthemall and that is it cannot find the link correctly of the .flv file that it is supposed to download.
So that is why you would need Adblock-Plus extension in your Mozilla Firefox. This would let you to pick the flv file when you visit the video's page. When you go to the video's page for example this: http://www.youtube.com/watch?v=QV47oWxLpio&feature=channel_page then you would have to go there and then open up your Adblock-plus which would be at the bottom of the page and then you would have to press "End" in your keyboard. It would take you stright down to the bottom part of the listings of the page that could be blocked with Adblock-plus.
When you get there, you might have to look around the bottom for the flv link would would say something like video.google.com or videos.google.com till this day. It might change later on. When you get that, you have to copy the link by right clicking the link in ad-block and copying it. When you are done copying, just paste it to your Downthemall and your can start downloading at a great speed.