SD cards have speed class ratings defined by the SD Association. 
The SD class ratings specify the minimum write speed for the device based on "the best fragmented state where no memory unit is occupied":

  Mark
Minimum Serial Data 
Writing Speed
SD Bus Mode Application
UHS Speed Class        UHS Class 3 30MB/s
UHS-II
UHS-I
4K2K Video Recording
UHS Speed Class        UHS Class 1 10MB/s
UHS-II
UHS-I
Full HD Video Recording HD Still Image Continuous Shooting
Speed Class 10MB/s High Speed Full HD Video Recording HD Still Image Continuous Shooting
Speed Class 6MB/s Normal Speed HD and Full HD Video Recording
Speed Class 4MB/s Normal Speed HD and Full HD Video Recording
Speed Class 2MB/s Normal Speed Standard Video Recording

 

UHS Speed Class is designed for UHS** devices only and designates minimum writing performance to record video on UHS products. The UHS Speed Classes defined by the SD Association are UHS Speed Class 1 (U1) and UHS Speed Class 3 (U3).

* (Speed Class and the UHS Speed Class are two different speed classes.)

**UHS (Ultra High Speed), the fastest performance category available today, defines bus-interface speeds up to 312 Megabytes per second for greater device performance. It is available on SDXC and SDHC memory cards and devices.

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