It isn't slow enough for me not to notice while tinkering with it. Of course, I'm running it a kvm vm and have 8 cores of 3.5 ghz going on. It would be hard for a terminal environment to be slow on that.

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.