This is an official application of Youtube that can download videos directly from youtube player similar from your native youtube player just click the more option and you'll see a download tab option. This is already tried and tested by me using my Samsung Galaxy y phone and it has a good speed in terms of streaming and downloading . Actually it really looks like the ordinary native youtube player in a much more smaller kilobytes, it only has 644kb and because of that I already uninstalled my internal youtube application using root uninstaller.
Download: http://adfoc.us/309556368936