Still trying to figure out what exactly is going on here, I took somebody else's advise and tried to see if maybe there is a relation between the size of the zpool and the NFS performance.
Connecting the machine in question to a different XRaid with a 745 GB Raid-5 disk, I tried to create a single zpool on that disk. Again, the same performance problems as noted earlier. Then I partitioned the disk into a 100 GB partition and tried to create a zpool on that. Again, no luck. Performance still stinks.
Related patches and bugs:
July 31, 2006