Last week the UK journal PC Pro published an interesting article about fast SD cards http://www.pcpro.co.uk/features/380167/does-your-camera-need-a-fast-sd-card, with a good description of the SD card Class system. With some clever testing, they show how six cards perform in a continuous shooting situation.
These tests also demonstrate how the SD card manufacturers have customized their firmware to handle sequential write cases. A class 10 card requires a minimum of 10 MB/sec throughput, and a supplemental rating system for Ultra High Speed (UHS) indicates a higher clock rate and correspondingly higher transfer rate. For the larger frame sizes (12 megapixel photos, HD video) high transfer rates are a requirement. The resulting data is almost always sequential, which matches the firmware characteristics well.
This article brings out one more interesting point. The authors point out that the performance measurements from using an SD card in a desktop system don't always reflect the use case. They end up performing their tests using an actual camera, thereby getting as close to the use case as possible.
For an application which uses random I/O (such as tablets and other Android devices), these firmware optimizations aren't necessary. In some cases, such optimizations actually lower random I/O performance. Similar firmware shows up in eMMC media as well. A software solution (such as FlashFXe) can adjust much of the I/O to be more sequential and more closely match the optimized performance. At Embedded World a few weeks ago we recorded our demonstration showing the benefits of our new FlashFXe product on eMMC.