Well...even if the entire timeline has been rendered (green bar), it will still need to be "rendered" or transcoded again to the final export format of choice. The benefit of using rendered previews is that if any of the clips has render-intensive effects that took a long while to render, you will save that extra time. Using previews for final export is ok for making a "preview" to look at. When doing the final export for delivery, best to not use previews to avoid quality loss and recompression.
Here's some info about render bars -
http://blogs.adobe.com/premierepro/2011/02/red-yellow-and-green-render -bars.html
Not a Mac user, but I think the way Final Cut worked was that pre-renders were in QuickTime, and if exporting to the same QT format, it did not re-render and simply repackaged the QT files together, so it was very quick (or so I read). With Adobe, even if exporting to the exact same format that the clips on timeline are, they will still be transcoded. My exports are nice and quick on Core i7 PC, even for H.264, but laptops (especially a few years old) will take a lot longer unfortunately.
Nice thing with Adobe is that files do not need to be transcoded before you start editing, just edit native files, so maybe the time you save on the front end you make up later.
Thanks
Jeff Pulera
Safe Harbor
