i think it may be a data rate issue. if you render to an uncompressed or ‘lightly’ compressed codec you will create large files (large as in file size). this requires the disk to be able to read a large amount of data per second (the data rate). when your file exceeds the data rate that can be read by the disk, then your computer/software will usually do one of two things… play the clip slower (looks slo-mo, not realtime) or play at a lower framerate (plays every 5th frame or so, and looks choppy).
most users will render to uncompressed or losslessly compressed codecs as an intermediate codec, then convert those to their working codec when they import into another piece of software, like premiere pro. so, if you were editing dv in premiere, you would convert the uncompressed file to dv at import, to give you the realtime playback. however, depending on your needs and system, you can just render straight to the codec that you are editing in, like dv, etc. you have to be careful about compressing and recompressing your media, but you can save a lot of disk space this way.
Kevin Camp
Designer – KCPQ, KMYQ & KRCW