I just started working with Vegas and I noticed something odd. Progressive renders are three times faster than Interlaced renders. Why is this? Also if my intended media is a DVD is there any reason not to use Progressive when outputting.
In general, if your source files are interlaced you will get the best results by staying interlaced all the way (unless you are intentionally trying to create an effect). Interlaced video updates fields at 50 or 60 Hz, so movements will tend to be smoother than progressive can provide at 25p or 30p.