qemu-img snapshot -c is unreasonably slow

Something fishy is going on with qcow2 internal snapshot creation times.  I don't know if this is a regression because I haven't used internal snapshots in the past.

QEMU 1.4-rc2:
$ qemu-img create -f qcow2 test.qcow2 -o size=50G,preallocation=metadata
$ time qemu-img snapshot -c new test.qcow2
real	3m39.147s
user	0m10.748s
sys	0m26.165s

(This is on an SSD)

I expect snapshot creation to take under 1 second.