They're improving different aspects that contribute to VM overhead:

> ...and boot in around 1ms on top of the VMM time (total boot time 3ms-40ms).

Firecracker is trying to minimize "VMM time", while the unikernel is minimizing guest overhead.

You can think of the VMM as the toolstack that executes the moment you send a command to start a VM up. Once it's done it hands over execution to the actual VM (i.e., the OS within the VM). Thus the total boot time is the sum of the VMM plus the actual VM boot time. In the past, both of these took a long time. With the advent of unikernels, which can boot in as little as a few milliseconds, the focus has also been on reducing VMM time.