Sorry - this is *not* new information. In Canon's defense this is all explicitly stated on page 117 of the owner's manual. If you are concerned about burst mode then leave camera on Standard or Auto Switch Card. The manual states: "When Rec. separately is set, the maximum burst will decrease greatly. (See pg 121)". Pg 121 further explains on the matrix the burst rates and also states: "The Maximum burst rate applies to High Speed Continuous Shooting", it then extrapolates that it is referring to UDM7. It may seem obvious then since the SD cads (if engaged in the two other record modes modes) would have to be the lowest common denominator. On page 32 the manual also states the camera does not comply with UHS standards although these cards can be used.
Now a good work around to this if in the field and you need the backup feature but also the fastest burst mode on a UDMA7 CF card is set the camera to record only on the CF card (Standard Mode). Be sure the CF is the card chosen to record (under Record/Play menu) as well. Now you can copy your images to the SD card by pressing Menu. Choose Menu section 3, sub menu 1 and select "Image Copy". Here you can define your Source (the CF card) and the freespace on Card 2 (SD card) there you can copy all your CF images to an SD card without losing burst mode speed and still have your backup.
revup67, excellent post! Thanks.
To the disgruntled, all you have to do is buy the 1D X instead. "Problem" solved--for just a few thousand dollars.