@blackdog: YT limits speed to actual required bitrate of the video. they want you to watch it
20kb/s sounds like the audio track at the end of the download as video is processed first. Only way to increase is to use more multiple connections.
@Grimzahn: Glad to hear that I could be of help to you
@plip: account login is broken/unstable and only required for some age restricted videos. no other advantages.
You might wanna try to use packagizer rules and set higher number of connections via rules to those links. But first try it that helps/works with rightclick on the link and there you can set custom chunks