If we’re talking about cold boot times, wouldn’t the relevant metric for unikernels be the hypervisor’s boot time?

How would that compare with containers running on Firecracker or other virtio-based μVM's?

A unikernel on Firecracker is probably going to start faster than a container on Linux on Firecracker.

I assume they meant using an OCI image for the rootfs of a firecracker VM, not running a container inside a firecracker VM.

Still difficult to see how the unikernel could be slower, but I doubt the difference would be huge? Don't have anything to back that up though.