Hi, Working with HDV files likw m2t or AVCHD are hard on the CPU as threir highly compressed. Just a few tracks and a few effects will lower your preview rate drasticly. Preview, auto is a good place to leave it. Also there is a roll off point on faster cpu’s helping here. We upgraded from a duo core to a faster quad core and saw no real difference it the preview window. But now the render time decreased by almost 400%. You can though, convert your HDV files to Cineform avi, a good lossess codec included with Vegas 7 or later I believe. Vegas works very efficent with this format and it can handle multiple render vith very little loss compared to m2t ot avchd. Hope this helps, Danny Hays