Saturday 6th April 2013: 11.56pm. Following on from last night's post, I fired that Sandisk microSDHC Extreme Pro into the SDCard slot of my Atom netbook for comparison. Got 19Mb/sec sustained read/write. That suggests that it's the SD card physical interface itself which is the bottleneck. Interesting then that a BB10 device at work gets 40Mb/sec read speeds with this card, I must remember to look into what SD hardware spec it supports.
For reference, and mainly because this information is nigh on impossible to find, here are the maximum possible speeds of SD card hardware revisions:
v1.01: Normal Speed (25Mhz) max 12.5Mb/sec.
v2.0: High Speed max 25Mb/sec.
v3.0: Ultra High Speed I max 25Mb/sec (SDR50, 100Mhz) or 50Mb/sec (DDR50, 100Mhz) or 104Mb/sec (SDR104, 208Mhz).
v4.0: Ultra High Speed II max 156Mb/sec or 312Mb/sec.
I should probably explain UHS-I, as it's a mess of incompatibility and it's far too easy to accidentally buy a component which bottlenecks your otherwise fast and expensive purchases. Specifically, to get your 90Mb/sec from your Sandisk Extreme Pro, you very specifically need a card reader marked as UHS104 in addition to UHS-I because the reader needs to run at 208Mhz, and most don't. You also need it to be USB 3.0 and not a reader which pretends to be USB 3.0, but can't shift more than 50Mb/sec in practice due to using a crappy controller. Ditto of course goes for your motherboard: as we saw, the Samsung Exynos ARM SoC's USB 3.0 controller can't push 50Mb/sec, and neither can the Renesas USB 3.0 controller on my workstation motherboard.
Things are a bit better with micro sd cards, because DDR50 support is mandatory only in the micro sd format. This has the paradoxical consequence that micro sd UHS cards are often twice the speed of full size SD cards e.g. Sandisk Extreme Pro (90Mb/sec) comes in micro and full sizes, but Sandisk Extreme (45Mb/sec) only comes in full size.
Anyway, for my ARM Chromebook, given it obviously only supports the max 25Mb/sec standard in its SD card slot, I therefore need whatever 64Gb SDXC card available which can deliver 20Mb/sec or so sustained write speeds. No point wasting a 45Mb/sec card on this machine!