Very confused as well!
The CF slot will support UDMA 7, yet the SD slot won't support UHS-1?
As you say, that defaults a 90MB/s UHS card down to Class 10 speeds...
Apparently, and this is just my opinion, the CF slot is definitely the primary...
The SD slot is there for back-up, WiFi, JPEG's, etc... But it will not match the CF slot for speed...
But it could have! That is the strangest past...