slow lvm

Hi, I've been having problems with a really slow lvm for a while. I think it started after I had replaced a broken hard disk and done pvmove. I've checked the drives with hdparm and they all seem normal. I've moved all the drives to a similar box and it's still equally slow. I've even added a new drive and created a lvm just on that and it still is very slow and just shows 100% utilisation of the drive but very little throughput (max 30k blocks/second according to munin).
I think the only think all of the configurations that I've tried have in common is that I started off with a linux software raid 1 with one missing drive. So here's my disk layout.

md4 : active raid1 sdd4[0]
      1885150680 blocks super 1.2 [2/1] [U_]

  --- Logical volume ---
  LV Name                /dev/vg0/lv0
  VG Name                vg0
  LV UUID                BEGmO7-Oi4O-DID5-4qs4-pvMp-1nnD-sHDz53
  LV Write Access        read/write
  LV Status              available
  # open                 1
  LV Size                1.76 TiB
  Current LE             460241
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           253:1

This is a fresh install of Debian squeeze. Before changing the disk it was Lenny. Any help really appreciate. I've spent about a month on this now.

Thanks,

mimo