...I think I used SD slot on mine once, had some shoot with slow enough stuff and must have pics, so set to record on both.
People are confusing SD Card performance and Canon 5DIII write/buffer performance. SD Cards are painfully slow on my 5D III, but when I put the same card in my 7DII the performance is indistinguishable from the CF Card.
People are confusing SD card performance on someone other's gear with SD card performance on their gear. I own a 5d mkIII and SD card performance on this body is not good. Even if an SD card can fly to the moon in a spaceship, that doesn't help me.
People forget that we're discussing a spec list of the 5D IV, extrapolating the SD write performance of the 5D III and assuming it will be a bottleneck also in the IV is nonsense. Unfortunately, SD speed in the 5D III was a well-known drawback, but, as unfocused noted, the 7D II SD write speed is much higher than that of the 5D III, we can reasonably expect the 5D IV to be even better than the 7D II in this respect.
Further, reliability of SD card is inferior to CF card reliability and technology wise in my opinion a compromise on a 5d body...
Reliability of SD cards is no longer a problem. Given the possibility to perform an in-camera low level format, deterioration of SD cards performance over time is no longer an issue, too. Concerning compromises, every single aspect of camera design and engineering is a compromise; unfortunately for the 5D III users, the SD card was a big problem for many, and using SD cards faster than 45 MB/s was of no benefit, so I can understand the annoyance of many. But I'd be optimistic for the future in this respect.