Threads and processes both require a context switch, but on posix systems the thread switch is considerably less expensive. Why? Mainly because the process switch involves changing the VM address space, which means a TLB shootdown: all that hard-earned cache has to be fetched from DRAM again. You also pay a higher cost in synchronization: every message shared between processes requires crossing the kernel boundary. So not only do you have a higher memory use for shared structures and higher CPU costs for serialization, but more cache churn and context switching.

it's all serialization - but that's not a bottleneck for most web servers.

I disagree, especially for a format like JSON. In fact, every web app server I've dug into spends a significant amount of time on parsing and unparsing responses. You certainly aren't going to be doing computationally expensive tasks in Node, so messaging performance is paramount.

i'd love to hear your context-switching free multicore solution.

I claimed no such thing: only that multiprocess IPC is more expensive. Modulo syscalls, I think your best bet is gonna be n-1 threads with processor affinities taking advantage of cas/memory fence capabilities on modern hardware.

this is the sanest and most pragmatic way server a web server from multiple threads

What is this I can't even.