Cause level of load of processing scales writes to cards. I can get continuous 1920*1080 @ 30fps with CF+SD, only when I disable pretty much everything, including rec blinking on LV. It is impressive (compared to what was otherwise), but not suitable for everything. When GD is on, combined CF+SD isn't that much different from CF alone (both fastest available). The definite advantage of CF+SD is max recording time - not terribly better, but always there.