@Wlad81
What settings are you use? Bit depth, fps, are you testing without global draw?
Is 5.4K anamorphic working (23.976fps) without card spanning? - > for me up to 14 bit continues.
Maybe fake CF card? Try UHD mode at 10 bit, 3840x1536, 23,976fps, with card spanning and global draw to off - > writespeed is up to 131mb/s possible with both cards, you can see the speed while recording.