All new SDCARDs conform to one of the speed classes defined by the specification, it’s usually stamped on top of the card and they are quite relevant for multiple reasons.

1. It defines the speed at which you can clock the card, 50MHz for class 10 and otherwise 25MHz.
2. Also alludes to the RU size, higher class cards pack more internal buffering.
3. Finally its primary purpose is of conveying the maximum write speed, class 10 being the fastest.

SD Bus Clock

High clocking capability means better internal circuit and more the bill of material$, class 10 cards are said to be capable of high speed (50MHz) mode, at least that is what the spec claims but the scriptures may not always reflect the reality. Running an intensive I/O test showed that in practice even a 44MHz clock tends to choke most of the cards, actually two out of the four popular makes exhibited inconsistencies. Even though card’s CSD register mentioned 50MHz, the general card behavior tends to be intolerant to intensive use cases at higher clocks, sometimes even before switching to high speed mode (via CMD6) the CSD register claim 50MHz compliance which seems contrary to the spec and when practiced bricks or crashes the card. The root cause of the high speed non-compliance is not clear to me but the fact that certain cards are more tolerant than the others should be kept in mind while testing the system.

Buffering

Internal SDCARD buffers expedite I/O by implementing a producer-consumer like scenario, Class 4 cards tend to have an internal buffer (RU) size of 32K while for class 6 it goes up to 64K and for Class 10 a whopping 512K, not surprising that the price of the card also follows a similar pattern, runs from $5 to $10 & to around $30 for high speed cards. Optimal use of cards would be to write in terms of the RU sizes so that the card buffers are utilized to the maximum, writing more than the RU size will make the host unnecessarily wait for the card to flush data to internal flash and transmitting very small packets will only keep the SDCARD firmware unusually idle. Performance depends on the depth and also on the maximum utilization of available computational units within the pipeline. So, with regards to SDCARD also, we need to simultaneously engage the host and the card which eventually leads to better throughput.

Write Speed

Time taken to write 128 MB of data was sampled over a dozen cards, Class 10 PNY card was the fastest at 17 secs while the slowest was 88 secs for the Transcend class 4 card. Below we have a graph which shows how the time to write is inversely proportional to the cost and the RU size and the speed class of SDCARDs. Embedded host used for this exercise was an Embedded  Artist EA 3250 board and it runs a 266MHz NXP controller.

Inference

Accurate class differentiation emphasized by the spec gets a tad obscured when it trickle down into the eventual product which seems to vary in behavior and performance capabilities across and within the same class. The challenge with SDCARD host productization is primarily that of interoperability testing, here we have considered only a dozen makes which is popular in the US, undoubtedly once we account for the China market there will be little semblance of sanity left in the above graph.

More on this topic – An account on SDCARD internals.