This is : How to play streaming Videos from Youtube with seeking enabled.
First , please laugh away my wrong and bad English (aka Japanglish).
I found mplayer-rc2 can play flv-video-streams with seeking.
so, I modded mYTube(one of the greatest apps, I think), wrote a simple script, and packed them.
yeah,streaming (not saving to a file) is the fastest way to play youtube.

1.it depends on mYTube (0.1.5-1).
2.download <mytube-streaming-plus_0.2_armel.deb>(http://sites.google.com/site/tamaneg...attredirects=0).
3.install it as root "dpkg -i mytube-streaming-plus_0.2_armel.deb"
4.start mYTube and tap <PlayST> instead of <play>.
5.a few seconds later , playback will start.
6.once you remove this package by "dpkg -r mytube-streaming-plus"
, mYTube returns to the original condition (all settings remain).

mplayer-rc2 comes from This thread.
I'm sorry for defiling codes of mYTube.
Thanks all the developers.
and RMS ,forgive me my license violation. Amen.