I can tell you what the problem is right now, actually. ZFS performs very poorly on low performance CPUs (i.e. your Atom N330). Try the same system with a different CPU and you'll get a different result. It's not a lack of bandwidth on your bus, memory, or disks, and it's not exactly the checksumming either (although that certainly contributes to the bottleneck), the grouping of transaction groups to be flushed to the disks is what your bottleneck is. I had an issue with some fairly poor performance with a Sempron 2800+ 45W TDP CPU and replacing it with an Athlon X2 did wonders.
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