Take the SD card out completely. It was discovered that when an SD card is in the camera, the CF card will record only up to the max rate of the SD card. Oh so I've been led to believe.
That's only if you're recording to both slots at once. If you specify a slot to record to, then it will utilize that card to its full potential. I have a CF and SD in my 5D, but have the camera record RAW only to the CF card. The camera will only fall back to the SD when the CF fills up. I haven't had any problems with card speed using this setup.
It's useful as an, "oh sh-t, I don't have another card." You wouldn't think it happens a lot, but it happened to me when I was in the Caribbean, and grabbed my camera for some sunset shots. Didn't bother taking my bag, just cam and tripod, and completely forgot to check my available shots. Having the SD saved me a 10 minute walk back to our hotel room during a nice sunset. Yeah, UHS would be nice... but even with my 7D I'd likely only use an SD slot as a backup anyway.