It's well documented that using a SD card along with a fast CF card can impact thre speed of the CF card in the 5DMKIII. The SD card slot has a max write speed much lower than the CF slot, and if writing to both at the same time the CF will slow down to match the SD.
A lot of people don't notice this but it's worth remembering if you do use both.
Having a SD in the slot but not writing to it will not impact performance.
I keep a SD card in the slot as a buffer in case my CF gets full, but I also copy a lot of shots to the SD from the CF card if it starts filling up. You can do this in camera so it can be a handy feature of you have a bit of down time when out and about.