Sony Vegas 11 playing very slow x264 videos format in the preview window

Hello.
i have a problem with Sony Vegas 11 on the preview window with x264/h264 videos..

They are playing very slowly and with lots of dropped frames and it is almost impossible to work this way ..

This does not happen with videos that their codec is "Lagarith Lossless"
but i can't store so big files...

i need a solution please !

Thanks for helpers!