This is the result of an experiment with memory cards and two different cameras, a Nikon D3 and a Nikon D3400. I wanted to know the effect of card speed on buffer unload, and actual on-the-field burst frame rate.
Buffer is fast memory, its size is fixed, and the number of photos it can contain only depends on the photo files size. The process is quite simple: the camera can take photos at its max frame rate until the buffer is full, then it slows down and can even stop taking photos while the buffers is transfered on a slower memory card, like an SD card or CF card. Actually, as soon of the first picture is taken, the buffer starts unloading into the memory card. That means the buffer is filled with new photos and unloads at the same time. In theory, if the internal memory transfert circuitry and the memory card were fast enought, it would be possible to have a burst max frame rate lasting for as long as you keep the shutter button pressed.
Official specs say the D3400 buffer can hold 8 12-bit compressed raw photos, or 100 large fine jpegs.
The D3 buffer can hold 18 12-12bit lossless raws, 16 14-bit lossless raws, 20 12-bit compressed raws, 16 14-bit compressed raws, 17 uncompressed 12-bit raw, 16 uncompressed 14-bit raws or 52 large fine jpegs.
To measure the effect of memory cards on burst frame rate slow down, I put each camera in fastest burst mode, arbitrary lowest isos, F1.8 and 1/1000 speed. The Nikon D3 was set to 14-bits lossless raw, and the D3400 was set to 12-bit compressed raw. Any image treatment was deactivated. Both Cameras were hand held shooting a static object, AF locked so it didn't influence the frame rate, and I recorded the shutter sound with audacity.
With a 16GB Sandisk Ultra 50MB/s, the D3 took 16 photos at the max frame rate in 1.79s @ 8.93FPS. Then it slowed down and took 20 photos in 14.6s @ 1.36FPS.
With a 32GB Sandisk extreme pro 160MB/s, the D3 took 16 photos in 1.77s @ 9.03 FPS. Then it slowed down and took 20 photos in 6.59s @ 3.03FPS.
As you can see, in this case, a faster memory card clearly improves the burst rate once the buffer is full.
With a 16GB Sandisk ultra 48MB/s, the D3400 took 8 photos in 1.56s @ 5.12FPS, then slowed down and took 20 photos in... 40.1s @ 0.49FPS. Painfully slow.
With a 32GB Sandisk extreme pro 95MB/s, the d3400 took 20 photos in 3.95s @ 5.06FPS, then slowed down and took 20 photos in 5.35s @ 3.73 FPS.
In this case, a faster memory card improves the apparent buffer size by 12 photos, because the buffer unloads so fast in the memory card that it takes longer to fill it up. Actually, the buffer unloads just slightly slower than it fills up. Here, the improvement is quite dramatic.
I also experimented with the D3 set to shoot at 5FPS. It shot 27 photos in 5.21s @ 5.18FPS before slowing down and shooting 20 photos in 6.56s @ 3.05FPS with the Sandisk extreme pro 32GB card.
Since I shoot quite a lot of wild life, especially birds and insects, fast frame rate and long bursts are something of importance to me. Obviously, a faster memory card can have a huge effect on the burst duration and frame rate once the buffer is full. It doubled the frame rate once the buffer is full on the D3, and the improvement was even better on the D3400. For a few more bucks, a fast memory card is definitely a no brainer.