I have a 5D3 and 32gb CF cards. I wish to have a card in the SD Slot and be able to switch from stills on the CF and Video on the SD. There are comments on the internt that by having an SD (SDXC) writing to either card is limited to the slower rate of the SD card. Is this true and if so, in the real world is the result likely to be of significance to the average bird photographer?