This could be to any number of reasons. I know that certain file systems have "copy-on-write", including Ext3, ZFS, WAFL, VERITAS (NetBackup), and btrfs. However, while COW (copy-on-write) COULD slow your system down, it should not be something easily noticable.
Also, if you have snapshots enabled (with whatever controller you are using to create the LV), that would also slow them down.
Depending on the size of the physical extents in the volume, it could slow it down (say, perhaps, that each physical drive was of a different total volume). Or perhaps something is going on with one of the individual physical disks that make up your LVM.
Try to do a few diagnostics on the disks themselves and see if any of them return bad sectors/slices. Are you growing this logical volume automatically, or is this a fixed-size volume?
It could just very well be the way that your LVM mirror is set up. The controller, or physical disks, may have something going on. You also have to remember the speed on the controller, since the path that data takes to hit the drives goes through the controller - having an older controller with newer HDDs doesn't speed things up (depending on the older controller, i've got one that is ages old and almost always outperforms some of the newer ones).