I really like how Plan 9 solved communication through sockets and files (which is how the OS doesn't care if any part of the system is remote or local). Just sayin, it has been done pretty elegantly.
But does it run efficiently and quickly?
I'd aruge: not slow enough not to be worth the benefits of userspace everything, the ability to have a completely distributed system, and not slow enough not to make the interaces with the kernel not wonderful. I much like how plan 9 skips the whole RPC and syscall nonsense and does almost straight file IO and socket communication for everything.
It is like Java vs C - it isn't slow enough not to take the niceties in some circumstances. High performance computing might like the Linux kernel, but as a developer who wants working applications that are really easy to interface with OS features, Plan 9 has that nailed.
I don't know where to start.