Abysmal I/O with KVM guest

Question

Setting up my home server with KVM so I can replace several machines with it. All has been going well. I have three guests with root partitions set up on qcow2 image files. One machine I want to run a 500GB samba share on. I would rather the files be stored on a partition directly on the hard drive rather than in a qcow2 image. I've set up a partition as a linear raid array as described here.
This works well and I'm able to mount it correctly. The problem comes when I try to transfer files on the guest. The transfer rate is terrible. I have this set up on an encrypted disk with RAID 1 (hardware), so I'm not expecting it to be blazingly fast, but it's still terrible even by those standards.
Here's rsyncing a 1GB file inside the guest on the 500GB partition:
sending incremental file list
test.img
1048576000 100% 3.33MB/s 0:05:00 (xfer#1, to-check=0/1)
sent 1048704084 bytes received 31 bytes 3478288.94 bytes/sec
total size is 1048576000 speedup is 1.00
This is transferring a 1GB file on the qcow2 image (root) of the guest:
qcow2
sending incremental file list
test.img
1048576000 100% 30.50MB/s 0:00:32 (xfer#1, to-check=0/1)
sent 1048704084 bytes received 31 bytes 32267818.92 bytes/sec
total size is 1048576000 speedup is 1.00
5GB file on the host (unencrypted):
Host on root partition
sending incremental file list
test.img
5242880000 100% 29.62MB/s 0:02:48 (xfer#1, to-check=0/1)
sent 5243520085 bytes received 31 bytes 30935221.92 bytes/sec
total size is 5242880000 speedup is 1.00
Mounted the same 500GB partition on the host:
sending incremental file list
test.img
1048576000 100% 32.87MB/s 0:00:30 (xfer#1, to-check=0/1)
sent 1048704084 bytes received 31 bytes 33292194.13 bytes/sec
total size is 1048576000 speedup is 1.00
Here's how I start the guest:
qemu-kvm -drive file=guest1.img,if=virtio,cache=none -drive file=/dev/md0,if=virtio -drive file=/dev/md1,if=virtio -net nic,model=virtio,macaddr=00:00:00:00:00:00 -net tap,ifname=tap0,script=no,downscript=no -m 2048 -smp 4 -cpu host -vnc :50000 &
How the RAID is created (used the defaults from the guide):
mdadm --build --verbose /dev/md0 --chunk=16 --level=linear --raid-devices=2 /dev/loop0 /dev/mapper/vg-network

Answer 1

The point of this was to keep the partition on the disk intact, while letting the guest access it. I first tried sharing the whole disk with the guest (`-drive file=/dev/sda,if=virtio`) and mounting the partition, but this was less than ideal as the disk is encrypted, so I'd have to type in my passphrase twice. Once on the host and once on the guest. With a software RAID, a small file is created to act as the MBR, then the RAID is aligned so that the partition on the RAID matches with the partition on the disk (so you can deactivated the RAID and still mount the partition).
I haven't used libguestfs and kpartx yet, but from what I've read, those are for accessing disk images, correct? What I'm trying to avoid is using a 500GB .img file.
I guess I could set up an NFS mount on the host, but then I might as well use that for my network storage. The idea here was to strip down the host so it's only doing one thing: KVM. Then the guests would replace the machines I have running now. One of those is running a samba share I use as network storage.