I tested my Nikon D800 with a SanDisk Ultra 64 GB 80 MB/s SD card.
I have a device which measures time and is triggered by the green LED light on the back of my camera which indicates that it's writing to the memory card. I took an image and measured how long the green light was on. I put the value into a table (shown below) and took the next image. I did this with 10 images and it seemed that the amount of time for saving the image increased with every shot. Here are my results (in seconds):
5.671 6.616 13.740 22.843 22.970 22.360 22.753 22.600 7.189 4.019
Between the green light going off and the next shot, I just put the value into my computer. This took me about 2 to 3 seconds. I can only imagine that the camera needed to clear the buffer or something like that in the meantime.
Maybe the green light switching off does not mean the camera finished all its work.
I was capturing uncompressed 14-bit RAW files. Saving times decreased to about 1 second using a decent CF card as the primary card.
Usually I use this SD card as a second card for saving low-res jpegs as a backup. I did not notice the same slow-down effects using this SD card as the secondary card.