This is a known issue with flash in linux, where the product delivered by Adobe is far lower quality than that of windows (already of questionable quality).
Efforts have been made to develop an open source alternative to the adobe flash player, but no definitive improvements with video are reached as of now (that i'm aware of).of).
That said, you'll have different experience depending on the video driver / kernel version you are using. For example here with a modest ATI Radeon HD4250 and the latest kernel, with the Gallium OSS driver the site you mention does play smooth.
