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Combustion stores everything in cache uncompressed. Every layer (depending on what you're doing).
You can also have severe problems with OpenGL previewing. I use software opengl as I've had less problems with it.
I
Try c3, the caching is "smarter". Generally you'll find combustion to be the slowest to render compared to similar desktop packages, no matter what the codec. Not a huge speed difference, but definitely slow sometimes.jon a