Memory card speed is confusing. There are different ways it is specified and advertised. The Read and Write speeds are different, NOT the same number (the camera writes, your computer reads).
There are three major types of memory cards used by cameras (each fits a different unique socket, NOT backwards compatible). Your camera has to be designed to specifically accept them, and their sub-type.
Class ratings (used by SD cards): Class 10 means 10 MB per second minimum, and it means the slowest of Read or Write will be at least the 10 megabytes per second. That will be Write speed, because Read is faster than Write. I don't know why Class 10 is still the highest number used, but it is a minimum (more recent faster cards have adopted the X megabytes per second ratings). Another factor is that many SD cards today have a UHS-1 interface, and are faster.
X ratings (used by Compact Flash cards): 400x means 400 times faster than the old first CD players which were 150 KB/second (which seems a pointless comparison today, but flash memory is older than digital cameras). But 400x means 400 x 150KB = 60 MB/second. This is Read speed, Write speed will be slower. Above 200x, the card will require a USB 3.0 port to ever see its full read speed.
Some of the faster cards have finally started just specifying the fastest speed as megabytes per second (MB/s). This will be maximum Read speed, Write will be slower (and your expected actual write speed will be difficult to determine when buying, and the speed in the camera is likely less than the speed in a computer).
There are now UHS-1 (Ultra High Speed) SD cards, and UDMA-7 Compact Flash cards, which are newer and faster, and are supported in newer cameras or card readers if so specified. These faster cards(of same type) are backwards compatible with older cameras, and vice versa. They will always work fine, they just run at the slowest speed supported. The card recommendations in your camera or card reader manuals will mention UHS-1 or UDMA-7 if supported.
SD cards have these size types (check the camera manual and card reader for types the camera supports):
Camera industry standards define a folder on the card named DCIM (Digital Camera Images). It will contain camera subfolders with names including three digit numbers, meaning that one folder will hold at most 1000 images, and then a new subfolder is created.
SD UHS U1 means Minimum write speed is 10 MB/second. UHS U3 (speed class 3) means Minimum write speed is 30 MB/second. The Nikon camera models (up through D750) support UHS-1. Write speed also depends on the device (or the specific camera model) doing the writing. For example, SanDisk advertises:
The XQD card is from Sony so far, and is much faster yet (new and still changing). Lexar was also making XQD until Micron closed their Lexar production (June 2017). The XQD card is used on some new cameras, and are significantly faster cards, finally able to handle writing longer bursts of large raw files. Your camera will specify which type it can use.
Point is, cameras have different processors and different card interfaces, and are also doing other work at the same time. And be aware that the actual speed also depends on the host device, and the camera likely cannot write full rated card speed. The speed may be pretty high, but something less than expected.
1000x is 150 MB/s, UDMA-7 maximum is 167 MB/s, and I see 125 MB/s read here (but same card setup on a USB 2.0 port starts with only Read 31 MB/s, Write 25 MB/s).
Copying a 3.6 GB file to 1000x card via same USB 3.0 setup, Windows 7 reports about 88 GB/s Write speed. Copying that file FROM the card to a system SSD drive reports about the same 125 GB/s. Galbraith tests Write speed of this 1000x card in a D800 to be 69 MB/s, and the same card in a D700 was 42 MB/s (so, situations will vary the numbers).
So is the card faster than the camera can use? Not necessarily. Maybe cameras won't see the maximum speed, but the faster cards still go faster than the slower cards. And the card reader speed is important for large batches too.
Video use does not need so much Write speed. Minutes of file might be large, but the camera has the full record time to write it. Nikon says use a Class 6 card or faster for HD video use (1080p), but they raise this minimum to 45 MB/sec for XQD cards (on cameras that can do 4K video). However, you certainly may want a card with much faster Read speed to download gigabytes of files to the computer.
Photographs: There is single shot mode, and continuous burst mode. There are large Raw files and smaller JPG files. All this affects the speed.
If you shoot JPG, a 24 megapixel image file might be 12 MB. If you shoot one JPG each second, that is only 12 MB per second needed for card speed. If you shoot continuous 5 frames per second, that is 60 MB/second.
If you shoot Raw, the files are larger, maybe 22 MB (24 megapixel). Then 5 fps becomes 110 MB/second, or 175 MB/sec. But that speed seems likely that the camera may not be able to match the card write spec, and if you do more than a second or two burst of continuous large files (whatever the buffer can handle), you probably want all the card speed you can get (but SD and CF cards are not that fast for continuous large raw files).
But if you just take one picture at a time, then card write speed is not much concern. Just watch the green LED on the rear of the camera (access light for the memory card), indicating when it is writing to the card. If that LED is only on for an instant, it is no concern. If it is on one full second after the shutter, is that actually important? Is it holding you back? The camera will still buffer a few more additional shots. So within reason, Write speed is probably not much concern to most of us. Unless you run off a long continuous sequence of large raw files, and then it surely is.
But note that when you Read hundreds of picture files into your computer, you will be thinking about a faster card and a USB 3.0 reader then. Or you could go get coffee then.
In burst mode, the camera buffers up the several files, which are written as it can. You can evaluate the performance this way:
Example: A Raw file on a 36 megapixel D800 is around 40 MB. If you shoot a burst of five of them, that is 200 MB to be written (plug in your own numbers).
A Lexar 1000x Compact card (150 MB/sec read, 95 MB/sec write) writes for 3 more seconds after shutter stops (200 MB in 3 seconds is about 67 MB/second). Ron Galbraith says the D800 can write 69MB/sec to this 1000x card (200MB/69 is about 3 seconds).
An old Class 10 SD card writes for 19 seconds after shutter stops (200 MB in 19 seconds is about 10 MB/second). Which would not likely be acceptable in this mode.
Writing five smaller 12 MB JPG files is only 60 MB total, much less issue (even within 6 seconds on a Class 10 card). But burst mode is when you will want faster card Write speed.
However if just shooting one 12 MB JPG once every second, you only need 12 MB/second card write speed. And again, the camera will still buffer any second quick shot then.
Summary:
Shooting 1080p video is not an issue for any Class 6 or 10 card. It has the full record run time to write it.
Write speed is not much issue for single photo shots. However, burst mode of large files is likely something different.
And did I mention that you always might want a faster card and USB 3.0 reader to download a few gigabytes to a computer? (which is not really about the camera.)
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