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The transfer speed of any adapter will always be slower than what it would be for the original card. The rate that data is transferred out of the camera is dictated by the SD host controller, any data it outputs will then have to be handled by the adapter, which can never go faster than the data it is being fed. I'm not sure what you're really asking though, do you want to use two SD cards, with one just in the CF slot? If that's the case then it might technically work if you got a really good adapter, but it's probably a waste of money. You probably just need a faster SD card. Magic lantern has a utility to test the data transfer rate of your memory cards. You could use that to see how fast it's actually working and if your cards are just shit.
