I've seen many people complaining about this problem - both here and elsewhere - and none of the responses so far have solved it for me, so that's why I'm posting a new thread. Anyways, I'm using Sony Vegas Pro 8.0c.
I'm quite sure that the whole problem seems to be coming from video formats rather than strength of the system because I'm using Toshiba Satellite A300-1MM, so there shouldn't be problems about that.
Anyway, the smallest edit will cause a lag - for example, I put a clip, split it, and set a transition between the two parts, and the video in the preview window starts lagging and gets immensely slow. I've been said that changing preview quality will help, but none of that worked.
I've tested different video formats and the lagging does vary respectively. Since I'm working with a .mkv format right now (which, I assume, cannot be imported to Vegas), I converted it to AVI (even tried various codecs) and the preview is unbelievably slow. Then, I converted the same file to DVD Video NTSC Movie (.mpg) using mpeg2video codec and it doesn't lag at all.
But that format doesn't allow me to have 640x480 resolution, which is precisely what I need before importing video to Vegas.
So, any ideas how to make AVI become smooth in a preview window?