In what sense is the video playing SLOW - Lag between video and audio or does teh video hang when it is playing ?
One more question, does this happen to all videos played on your computer or all videos played on the browser (Firefox, I suppose) or only on this website ?
If its for all videos played on your computer - check your Video card driver. I faced this problem and I added in NVidia driver for Ubuntu and all went fine. Google for "Install and Enable NVIDIA driver on Ubuntu 9.04" and go on from there. (THIS IS ONLY IF YOU HAVE NVIDIA AND ALL VIDEOS PLAY WITH A HANG/PAUSE).
If its for all videos on the browser ... It should be some plugins that needs to be upgraded or downgraded ...
If its only on that website, then its the problem with the website owner and nothing can be done