There is a permissions issue where /dev/kvm is not writable, and the VM's drop into qemu mode which causes the slowdown. You can verify that this is the issue with:

cat /var/log/libvirt/qemu/(internal_id).log | grep -i kvm

In the results, look for:

open /dev/kvm: Permission denied
Could not initialize KVM, will disable KVM support

Fix with:

chgrp kvm /dev/kvm
chmod g+rwx /dev/kvm