DusX speaks the truth. Use windows native codecs when you can -- much more efficient than QuickTime formats. But if you're scrubbing the video using the 'position' input, or want to play movies at a speed other than 1, you must use QuickTime as the Windows playback mechanism doesn't support anything other than forward playback and normal speed.
Also, Patrick, using 5 x 1280 x 1024 is going to tax any system, especially if the video is of a matching resolution. You may need to investigate an SSD drive to keep things running smoothly.
Best Wishes,
Mark