> With 9p, for every T message there's an R message that is waited on before the next T is sent.

This is entirely false.

The 9P protocol is asynchronous, can handle up to 16k outstanding requests, supports out-of-order responses, and includes support for arbitrary request cancellation,

The only thing thing that blocks to wait for an R message are blocking syscalls, but that doesn't stop other processes from sending further T messages.