Resize is done on the CPU on CS3. It is not fast.
Your best bet to make it fast is to have the Sequence at the same size as the final output and in Premiere drop the footage onto the timeline accepting that you'll loose the top and bottom 6 pixels (12 total).
If you change the pixel aspect ratio to not lose those 12pixels you'll
a: have to run the same resize code so you'll get that speed penalty
b: get nasty aliasing because you're now interpolating pixels.