Canon 5DM3 Dual Card Slots

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What's the maximum speed you can get from the SD Card slot when using one in a Canon 5DM3. From what I understand the 5DM3 isn't able to access the fast write speeds on new model SD cards so would be limited if shooting high burst rates as the card would buffer. So would it be a waste of money buying faster memory cards for the SD card slot, rather get a lower speed higher capacity card and save money as the speed isn't able to be fast speed of new SD cards isn't going to be accessed in camera anyway. The only benifit I could see is when it comes to downloading the images with a USB3 card reader onto a PC, then you could access fast speeds of the SD card?