Why do flash players consume up to 20 times more CPU than VLC player to view exactly the same video file?

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http://askubuntu.com – When I play a video (H264 MPEG4 AVC 320x180) online (in Firefox or Chrome) it uses to consume up to 100% CPU, but if I download it (exactly the file played) and use VLC - it takes just 5% (6% at maximum). Why does this happen and what can I do to make on-line players more efficient? PS: The video card is Intel GMA X4500HD. (HowTos)