Instead of labeling their memory cards in MB/s, some manufacturers choose to use “Times” ratings (e.g. 8x, 12x, 20x, etc…). While it’s pretty clear that a higher number indicates faster speed, what exactly is the number a multiple of?
The answer is that the memory cards are using the same transfer speed rating as the CD-ROM industry, with 1x being equivalent to the data transfer rate of an audio CD, which is 0.15 MB/sec or 1.2288 Mbps.
If a CD-ROM is read at the same rotational speed as an audio CD, the data transfer rate is 150 KiB/s, commonly referred to as “1×”. At this data rate, the track moves along under the laser spot at about 1.2 m/s. […] By increasing the speed at which the disc is spun, data can be transferred at greater rates. For example, a CD-ROM drive that can read at 8× speed spins the disc at 1600 to 4000 rpm, giving a linear velocity of 9.6 m/s and a transfer rate of 1200 KiB/s. [#]
Even though memory cards don’t use moving parts and have nothing to do with audio CDs, some manufacturers continue to use this measurement standard for labeling their cards.