If you're like me, you've spend the last year wondering why your KVM based virtualization is horribly slow when doing IO. You've tried twiddling every available option in your BIOS, you've mucked about with hdparm and libvirt/qemu settings, and you've run more timed installs of RHEL 5 than you care to admit.
If you're also like me, you tried using ext4 instead of BTRFS to store your guest images today, and found that this resolved your issue.
The relevant bz appears to be here.
The lesson I learned today is to blame experimental or newer features before anything else (though its doubtful I'll remember this next time).