Ok more info about speedtesting...

cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor will give you your current cpu governor setting

echo performance > /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor should peg the cpu to fullspeed
echo ondemand > /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor should set the n8x0 back to standard speed scaling
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My speedruns with 'performance' and Kingston 4GB Class 6: miniSD At 86% full capacity
57.85s / 13.31s
55.39s / 14.12s
39.02s / 13.24s
44.19s / 14.14s
42.37s / 13.40s

Best time:
2.56 MB/s write
7.55 MB/s read

Does SD get slower as it fills up?