Warning: Overclocking SD cards can corrupt your data and possibly damage your SD card!
If you want better SD card speed and have an UHS-1/3-capable SD card, you can make the Pi run it with more than the standard of 50 Mhz. I tried this with a Rasperry Pi 0 (the non-W variant) and a Verbatim 8GB SD-HC Class 10 micro SD card (Model #44081) and settled at 80 MHz to be safe.
Some results using Jeff Gerlings benchmark suite:
|SD clock in MHz||Read speed in MB/s|
|SD clock in MHz||Copy speed in MB/s|
|SD clock in MHz||write||rewrite||read||reread||random|
Enable SD card overclocking by editing /boot/cofig.txt and add:
# Overclock SD card. Check with "sudo cat /sys/kernel/debug/mmc0/ios" dtparam=sd_overclock=80
Make sure your overclock value is a fraction of your Pi’s core frequency (e.g. Rasperry Pi 0 core clock = 400MHz -> 400 / 5 = 80 Mhz). You can find the default core frequencies (“core_freq”) here.
You can print the current SD card hardware state with “sudo cat /sys/kernel/debug/mmc0/ios”:
clock: 50000000 Hz actual clock: 100000000 Hz vdd: 21 (3.3 ~ 3.4 V) bus mode: 2 (push-pull) chip select: 0 (don't care) power mode: 2 (on) bus width: 2 (4 bits) timing spec: 2 (sd high-speed) signal voltage: 0 (3.30 V) driver type: 0 (driver type B)