CompactFlash emulation speed is usually specified in "x" ratings, e.g. 8x, 20x, 133x. This is the same system used for CD-ROMs and indicates the maximum transfer rate in the form of a multiplier based on the original audio CD data transfer rate, which is 150 kByte/s, or .15 MByte/s. For example, 133x rating means transfer speed of: 200 * .15 MB/s = 30 MB/s. These are manufacturer speed ratings. Actual transfer speed may be higher, or lower, than shown on the card depending on several factors. The speed rating quoted is almost always the read speed, while write speed is often slower. – from Wikipedia 9/5/2013