On high-latency links, 9P is slow, but on low latency links (like should be provided by QEMU), 9P should be very fast. Looks like a QEMU implementation problem to me.
That being said, calling QEMU's 9P "9P", is a stretch. 9P was designed wit multiplexing in mind, but it's not multiplexed in QEMU, in fact multiplexing in QEMU uses a completely different mechanism. You can't mount QEMU's thing in Plan 9 (unless someone added support for this when I wasn't looking).
I wouldn't find it surprising at all if the fault lies with qemu's implementation rather than with some inherent property of 9p. The performance test I ran was with qemu's virtio 9p implementation: compared to a kernel compile on the host, one in the guest took a few percent longer on a virtual block device, or hundreds of times longer on virtio-9p.
That being said, calling QEMU's 9P "9P", is a stretch. 9P was designed wit multiplexing in mind, but it's not multiplexed in QEMU, in fact multiplexing in QEMU uses a completely different mechanism. You can't mount QEMU's thing in Plan 9 (unless someone added support for this when I wasn't looking).