It's an issue (Of Sorts) with the software Canon have put into the Camera regards the Write Speed for the Buffer system
The 5DMK III (And The 1Dx) will only write to the Cards vai the buffer at the lowest speed dependent on the Crds you have in the card slots, on the 5DMK III for example, I have a 128GB Sandisk Card with a write speed of 100Mbs, I also have an SD Card 64GB Sandisk with write speed 95Mbs, On my system I write to both Cards simultaneously in RAW, but wether your doing RAW (Slower because bigger files) or JPEG, the Camera will write at the speed of the slowest Card, in my case the SD Card @ 95Mbs, it's the way Canon have set the system up.
It's more noticeable on the 5DMK III only because Canon chose to put the Camera out with a CF Card slot & a SD Card slot.
On the 1Dx you can have the same issue if you insert two CF Cards with different Write speeds, i.e. Lexar 1000x Pro 128GB CF Card and the Sandosk 128GB 100Mbs, originally I thought these Two Cards had the same write speed, but in fact the Lexar is 30% faster, on Paper.
Only way for the 5DMK III to write at Fastest speed is to put a single CF card in & leave out the SD Card, I'm relatively certain that even with the SD card installed without any write to instruction, the camera will still write at what ever the slowest Card speed installed. I'm not positive on this as there does seem to be some differing opinions, but seems to make sense if you have an SD Card installed.
I live with the small write speed difference to ensure I have two complete copies on two different cards of every Image I take, I had one instance of a corrupted CF Card sometime ago and I'll not take the chance again.