Speeds
An SD card's speed is measured by how quickly information can be read from, or written to, the card. In applications that require sustained write throughput, such as video recording, the device might not perform satisfactorily if the SD card's class rating falls below a particular speed. For example, a high-definition camcorder may require a card of not less than Class 6, suffering dropouts or corrupted video if a slower card is used. Digital cameras with slow cards may take a noticeable time after taking a photograph before being ready for the next, while the camera writes the first picture.
A card's speed depends on many factors, including:
The frequency of soft errors that the card's controller must re-try
The fact that, on most cards, writing data requires the controller to read and erase a larger region, then rewrite that entire region with the desired part changed
File fragmentation: where there is not sufficient space for a file to be recorded in a contiguous region, it is split into non-contiguous fragments. This does not cause rotational or head-movement delays as with electromechanical hard drives, but may decrease speed by requiring the controller to do more computation[citation needed].
With early SD cards the speed was specified as a "times" ("
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A lot of reading
