It is not possible to achieve 17.5MB/s+ without overclocking SD bus.
Well, that doesn’t seem consistent with my results… I’m getting read speed of 21.3 MB/s with fresh install and read speed of 28 MB/s when I overclock. However there is virtually no change in boot time (26 secs).
Writes results are much slower and less consistent from test to test 4 to 10 MB/s.
Could there be an issue with the benchmark utility?
Use official power supply.
Did you solder the GPIO on RPi Zero? Can damage the board if excessive heat transferred for long periods.
I’ve tested this on two Pi zeros (one original and one brand new “W” that has not been soldered) with two identical SD cards and with various power supplies (including 7AH battery)