Here's my bit of uninformed speculation: presumably a lot more potential for aggressive optimizations. It's unlikely that you can make Linux as bare-bones as you could a unikernel. Besides, for all intents and purposes, you can't run Linux through a whole-program optimizer along with your application (not sure if this is even a worthwhile objective). I would also think that communicating between the application and the "kernel" via function calls instead of system calls would yield some non-negligible performance gains as well.