Why apps don't playback HD properly?
 

Hi Guys,
I think this is my first post. Thanks, great forum.
My query came to me recently when Vegas was struggling to play back some HD Clips in real time when Windows Media Player played the clip back perfectly, no dropped frames. Adobe Bridge will also play back the same material without a hiccup. Adobe Premiere Pro, same thing, struggletown. I can understand if there is a lot of processing to be done but the editing apps can't even play back a straight clip. We are told that HD is hardwork for a computer but obviously not always.
What gives?
Thanks.