The maximum write speed is based on a totally erased (new) card. Formatting a card does not erase it unless its a low level format. Then the card has to first erase a block of data before it can write to it, and for a SD card, its SLOW.
The 6D, like the 5D MK III will probably have a low level format function for SD cards which will totally erase a card, but it will take a long time, maybe 20-30 minutes depending on card size, so its best done in a computer where you don't run your battery down.