Why is a wedge on RAID 10x slower?

[Stephen Borrill <netbsd%precedence.co.uk@localhost>]

I've a RAIDframe RAID-5 on 3x WD40ERFX drives. There's a well-aligned gpt 
and wedge on each:

# gpt show wd0
       start        size  index  contents
           0           1         PMBR
           1           1         Pri GPT header
           2          32         Pri GPT table
          34          30
          64  7814037071      1  GPT part - NetBSD RAIDFrame component
  7814037135          32         Sec GPT table
  7814037167           1         Sec GPT header

# raidctl -G raid0
# raidctl config file for /dev/rraid0d

START array
# numRow numCol numSpare
1 3 0

START disks
/dev/dk0
/dev/dk1
/dev/dk2

START layout
# sectPerSU SUsPerParityUnit SUsPerReconUnit RAID_level_5
64 1 1 5

START queue
fifo 100

Underlying write performance on the raw RAID device is great:
# dd if=/dev/zero of=/dev/rraid0d bs=1m
^C19476+0 records in
19475+0 records out
20421017600 bytes transferred in 67.089 secs (304386972 bytes/sec)

If I create a similarly well-aligned GPT and wedge on the RAID device, 
write performance is horrible:
# gpt show raid0
        start         size  index  contents
            0            1         PMBR
            1            1         Pri GPT header
            2           32         Pri GPT table
           34           30
           64  15628073887      1  GPT part - NetBSD FFSv1/FFSv2
  15628073951           32         Sec GPT table
  15628073983            1         Sec GPT header
backup 50# dkctl raid0 listwedges
/dev/rraid0d: 1 wedge:
dk3: bigdata, 15628073887 blocks at 64, type: ffs

backup 51# dd if=/dev/zero of=/dev/rdk3 bs=1m
^C388+0 records in
387+0 records out
405798912 bytes transferred in 24.320 secs (16685810 bytes/sec)

I don't understand the disparity in performance.

--
Stephen