This article has pictures of gnome-disks benchmarks that was run on a GT-I9300 under Parabola with the Replicant 11 kernel and u-boot with the nonfree BL1.
Xorg had instabilities so I had to take pictures before the display went black and/or were corrupted.
This information might be interesting if you want to know if it's better to run an OS on the eMMC or MicroSD (for instance if you want to do tests and need to reboot often or want to compile on the device).
Here we can see that the read speeds are between 70 and 80 MiB/s in average
I tried various microSD cards that are supposed to go faster than 25MiB/s but they seem to be limited to 25MiB/s on the Galaxy SIII (GT-I9300).
It's unclear if the Exynos 4412 used could support faster cards:In one hand, one of the reference manuals of the Exynos 4412 says:
- Multimedia Card Protocol version 4.3 compatible (HS-MMC)
- SD Memory Card protocol version 2.0 compatible
Another Exynos 4412 reference manual says in the
Mobile Storage Host chapter:
The Mobile Storage Host is an interface between system and SD/MMC card. The performance of this host is very powerful, as clock rate is 50MHz and access 8-bit data pins simultaneously. This host supports 8-bit DDR (Double Data Rate) transfers. The specifications that Mobile Storage Host supports are: * Secure Digial Memory Card (SD Memory Card, Version 2.0) * Secure Digital I/O (SDIO - Version 2.0) [...] * Multimedia Cards (MMC - Version 4.41)
So if we have 50Mhz x 4bit (according to the devicetree we have only 4 bits here) x 2 (DDR) / 8.0 we could have up to 50MB/s but in practice we have up to 25MiB/s (DDR cards compatible with SD 2.0 probably don't exist or are not common).Various MicroSD (which should in theory be faster than 25MiB/s):