Look at your dmesg and any warnings about your drives that might appear in /var/log/message -- perhaps your drives are running in a low end DMA or PIO mode?
Or, perhaps you have multiple raid* devices defined on this pair of drives? Because the as-shipped /etc/rc runs "raidctl -P all", the parity-rewrite will be run simultaneously on raid0, raid1, raid2, etc. If these are on the same physical devices ... that would explain your horror story.
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