sadly Quicktime is broken beyond repair, i.e. you can not do HTTP range requests to allow seeking in videos beyond the part that is already downloaded. On top of that Apple knows about this problem but ignores it. They even went one step further and just fixed it for H.264 in Quicktime X. (note that they stopped supporting 3rd party components in Quicktime X)
Obscure, but that's kind of the point with this sort of app
On a sidenote: You can definitely resolve the speed issues people are talking / complaining about by pre-computing and storing everything, instead of doing it on the fly
I work part-time for a production company and as far as I know, the industry standard is the RED Camera http://en.wikipedia.org/wiki/RED_Digital_Cinema, whose standard output is 2K-4K RAW.
Compressed formats that interpolate between keyframes (like MPEG) are generally a bad idea since they make precise cutting between clips a lot harder.
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