There is a slight confusion amongst 5D3 users concerning write speed when both card slots are used.
It is true that in case you set the Camera to store on both Cards (Either both the same format, or RAW on one and JPG on the other), the total write speed will be significantly slower. But that's the Nature of the Beast that the out-of-time SD slot consists of.
(All thoughts and thought experiments here now under the assumption that both cards as as fast or faster than the maximum write speed the camera can perform!).
Let's assume we store raw on both cards. After the raw is recorded and pushed to the camera buffer, both slots now write to the respective cards. CF is faster, so finishes first. Now, there would be two possibilities for file handling: Either the CF system now stops writing until the "ok" from the SD operation is received, then the process starts again with the next picture which is already waiting in the buffer; or the CF already writes the next file from the buffer while the SD still works on the previous one. In both cases, the total write process is not finished until all shots in the buffer are stored on both cards and the buffer can be cleared.
But in case the SD has a card, but the Camera is set to write only one copy on the CF card, nothing like this happens, and the bust is stored as fast as if no SD card is present. You can try it at home, you wont see any difference unless you write on both cards parallel.
I use the SD slot as "Overflow" to prevent the Camera from stopping when my CF is full, and never see a slow down on bursts. At very important shootings I record toboth cards parallel, but then I have to live with slower burst speed, which often is not a problem.
