I would try to set up a minimal desktop environment and kill all unneccesary
processes; perhaps with a very fast video player it will work.
I recommend using xmonad together with mplayer.
So do a
apt-get install xmonad mplayer
then logout and choose "xmonad" as a desktop session.
You'll see nothing but a blank screen or maybe a wallpaper.
Hit "alt+return" to open a terminal.
Then type "mplayer /path/to/your/file.webm".
You can first try to checkout which processes are running with "top" and kill
everything redundant.
