So I just recently had the situation again that I was shooting RAW and fired away, ending up using the full buffer capacity and noticed that I had to wait quite a bit until it cleared fully. So, I checked this again and think I found out that, when you use the backup mode (write on both cards), the write times simply add up. As in: The camera doesn't write to both cards simultaneously. Is this correct?
I have a SanDisk Extreme Pro 95MB/s card in the SD slot, which has a real write speed of approx 64 MB/s (I also checked this with an app on my computer)
And a SanDisk Extreme Pro 160MB/s in the CF slot, it has a real write speed of approx 91 MB/s
When I use just the normal mode (overflow?), and it writes to one card only, it takes 10 seconds with the SD as the primary card and 7 seconds with the CF as the primary card for the buffer to completely empty again (the display shows 16 images capacity). When I use backup mode, it takes 17 seconds until the buffer is empty. I.e. the write times add up.
For those of you that have two memory card slots in their cameras, is this the same with your camera?