How much speed do you need?
A faster memory card doesn’t necessarily make your camera work faster, and there’s more than one kind of ‘speed’…
Continuous shooting: maximum possible speed A faster memory card won’t let your Nikon shoot at a faster rate. The frame rate is fixed by the camera hardware. But the camera is saving images faster than it can write them to the memory card, so it saves them temporarily to an internal memory ‘buffer’ while it’s shooting. The faster the memory card, the longer it takes for this buffer to fill up, and the quicker it can empty the buffer when it’s full. So a faster card will let you shoot a longer burst, and it will reduce the delay before the camera is ready to start shooting again. The maximum speed rating of the memory card is a pretty good guide to how well it’s going to do this.
HD video: minimum sustained speed
When you shoot HD video, the camera has to write the data straight to the memory card in ‘real time’. If the minimum ‘sustained’ speed falls below a certain level, you get dropped frames in your footage. This is why SD/SDHC/ SDXC memory cards now have a ‘Class’ rating and some CF cards have VPG 20 (20MB per second, fine for amateur use) and VPG 65 (65MB per second, for pros) logos. A minimum sustained speed of 10MB per second is adequate for 1920 x 1080 full HD video, while 30MB per second is needed for the new 4K video standard, though no current Nikon D-SLRs shoot 4K video.