NAND flash memory cards are much faster at reading than writing so it is the maximum read speed that is commonly advertised.
As a chip wears out, its erase/program operations slow down considerably, requiring more retries and bad block remapping. Transferring multiple small files, each smaller than the chip-specific block size, could lead to a much lower rate. Access latency also influences performance, but less so than with their hard drive counterpart.
The speed is sometimes quoted in MB/s (megabytes per second), or as a multiple of that of a legacy single speed CD-ROM, such as 60×, 100× or 150×. Here 1× is equivalent to 150 kB/s. For example, a 100× memory card gives 150 kB/s × 100 = 15,000 kB/s = 14.65 MB/s.
Performance also depends on the quality of memory controllers. Even when the only change to manufacturing is die-shrink, the absence of an appropriate controller can result in degraded speeds.
Read more about this topic: Flash Memory
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