SD Card speed class vs UHS speed

From their page:

SD Speed Class/UHS Speed Class

Greater Performance Choices

There are two kinds of Speed Class, "Speed Class" and "UHS Speed Class."

As a characteristic of flash memory, actual transfer speed varies. Variable speeds are difficult to reliably record streaming content such as video because it requires a constant writing speed. Speed Class and UHS Speed Class provide the constant speed necessary for video recording by designating a minimum writing performance so that minimum and constant speed is guaranteed for camcorders, video recorders and other devices with video recording capabilities under the conditional write operation specified in the standard.

Speed Class, designated as Class 2, 4, 6 and 10, is designed for normal and high speed bus interface (mode) and UHS Speed Classes 1 and 3 are designed for UHS bus interface*. (Speed Class and the UHS Speed Class are not compatible.)

*UHS (Ultra High Speed), the fastest performance category available today, defines bus-interface speeds up to 312 Megabytes

	Marks	Operable Under	Applications	SD Memory Card
Speed Class		High Speed Bus I/F	Full HD video recording HD still consecutive recording	SD, miniSD, microSD SDHC, miniSDHC, microSDHC SDXC, microSDXC
		Normal Bus I/F	HD ~ Full HD video recording
			SD video recording
UHS Speed Class		UHS-I Bus I/F UHS-II Bus I/F	Full higher potential of recording real-time broadcasts and capturing large-size HD videos (UHS Speed Class1 denotes a 10 MB/s minimum write speed)	SDHC UHS-I and UHS-II, SDXC UHS-I and UHS-II
			Capable of recording 4K2K video (UHS Speed Class 3 denotes 30 MB/s minimum write speed)