SD Speed Compatibility
It’s important to remember that Speed Class and UHS Speed Class are two different speed indication symbols for different devices. However, a UHS-I memory card may also indicate a Speed Class.

The host device Speed Class requirements are minimum requirements for optimum performance. If your host device requires a Speed Class 4 SD memory card, you can use Speed Class 4, 6 or 10 SD memory cards. If your host device requires a Speed Class 6 SD memory card, you can use Speed Class 6 or 10 SD memory cards.

Fragmentation and Speed
The memory of a card is divided into minimum memory units. The device writes data onto memory units where no data is already stored. As available memory becomes divided into smaller units through normal use, this leads to an increase in non-linear, or fragmented storage. The amount of fragmentation can reduce write speeds, so faster SD memory card speed standards help compensate for fragmentation.

The SD Association developed standards specifying various operating conditions for SDHC, SDXC, SDHC UHS and SDXC UHS memory cards to achieve the minimum speed according to the card's speed class. Speed symbols are provided to help identify cards that comply with these standards.