Because the still has to be converted to video, and it may be of any number of resolutions. Everything the still is has to be downconverted/converted to DV or whatever format you're rendering to. There are a lot of CPU cycles going on when you convert a still, and if you have stills that are .tif, they take even longer because beneath the hood, Vegas is calling on Quicktime to read the file, therefore slowing it down even more.