performance - Disk throughput dramatically reduced when using ZFS on OpenSolaris?

I'm building a simple ZFS file server for the small
business I work for. The server is a Dell Poweredge 840, with 1GB RAM. The OS
(OpenSolaris 2009.06) is installed on one SATA drive, and there are three other SATA
drives installed for storage: 1x1TB, 1x1.5TB, and 1x2TB. When I add the three drives to
one raidz zpool, throughput isn't very
good:

#zpool create -m
            /export/pool pool raidz c7d1 c8d0 c8d1
#zfs create
            pool/fs

#time dd if=/dev/zero of=/export/pool/fs/zerofile
            bs=1048576 count=1024

1024+0 records in
1024+0 records
            out

real 0m12.539s
user 0m0.002s
sys
            0m0.435s

That's
about 81.6 MB/s. That's not horrendous, but I tried creating a pool consisting of just
one of those drives:

#zpool create
            -m /export/disk-c7d1 disk-c7d1 c7d1
#zfs create disk-c7d1/fs
#time
            dd if=/dev/zero of=/export/disk-c7d1/fs/zerofile bs=1048576 count=1024
1024+0
            records in
1024+0 records out

real
            0m21.251s
user 0m0.002s

sys
            0m0.552s

Okay, 48.19
MB/s throughput for a sequential write to one drive? That seems pretty low. Especially
when I format the drive as UFS and try that same
write:

#newfs
            /dev/dsk/c7d1s2

#mount /dev/dsk/c7d1s2
            /mnt/c7d1
# time dd if=/dev/zero of=/mnt/c7d1/zeroes bs=1048576
            count=1024
1024+0 records in

1024+0 records
            out

real 0m10.372s
user 0m0.002s
sys
            0m1.720s

That's almost
twice the speed, 98.73 MB/s. That's much closer to what I'd expect out of these drives
(though they're just cheap SATA drives).

What
am I doing wrong here? I understand that there's overhead involved in writing parity
data with RAIDZ, but making a pool from a single drive shouldn't halve throughput,
should it? That seems pretty
bad.

Thanks,
everybody.