Currently Fuchsia test runner creates a minfs image that's passed to the VM to write test results. The problem with this approach is that the drive is exposed as AHCI device to the VM and it's rather slow under nested VM on GCE (writes can be slower than 1MBps). Test binary need to fsync() the file, before shutting down the VM. That fsync() often times out after 15 seconds, so it has to be retried, see  bug 796318 .

I see two possible ways to improve it:
 (1) Try using virtio driver instead of AHCI. It _may_ be faster than AHCI. Problem is that virtio is currently broken, see ZX-1309 (device fails to mount with this error "failed to allocate descriptor chain of length 128").
 (2) Save the file to memfs and fetch it from the VM using SSH. This requires forwarding SSH connection from the host to the VM, i.e. hostfwd= parameter for netdev config in qemu. That qemu feature wasn't stable in the past, but it work now (it worked when I tried it last week).

(2) is probably better if it works.