The Nikon D750 has two SD card slots that support fast UHS-I cards of type SDHC and SDXC. Cards with high write speed impove the performance of the D750. The UHS-I standard allows speeds up to a theoretical maximum 104 MB/s however actual write speeds in use may be lower.
UHS-I cards are denoted by the Roman numeral one “I” beside to the SD logo. A specific UHS Speed Class may also be present, which is the letter U with a number inside. U1 supports a minimum 10MB/s continuous write speed; U3 minimum 30MB/s write. (UHS Class is different from older SD Class, where the maximum is Class 10 supporting 10MB/s. The SD Class is a number inside the letter C. )
Because the Nikon D750 is capable of writing well above the Speed Class ratings, you would need to refer the SD card manufacturer’s claimed write speed. Many cards are only labeled or advertised with a read speed and the write speed may be significantly lower. Write speed has the greatest impact on shooting performance.
You can find a test of several different SD cards at fastest SD cards for Nikon D750. There is a difference even between UHS-I Class 10 rated cards.
The D750 supports UHS-I but not UHS-II. There is no benefit to using a UHS-II card in the camera, in fact some can perform worse than a UHS-I card. But a UHS-II card can allow you to download images from the card faster.
Recommended SD Cards for Nikon D750 — Fastest Speed
D750 SD Cards — High Speed
* Write speeds as stated by manufacturer. Actual write speed may be lower depending on device.