Do you use FCP on Windows?
See, FCP was designed for some codecs, but not all, especially not distribution codecs.
What software do you mean, playing back all your files in realtime under Windows?
PS: Final Cut Pro is a PRO application, using PRO codecs and not Xvid or Divx or other codecs, which were never meant for editing.
PPS: Those distribution codecs do not have all frames stored, only approximations and interleaves. Pro codecs do that too to some extent, but not in that same manner. So Final Cut has to render to a better suited format for so called RT playback.