Well, there's opportunistic nonblocking IO which you can do on your event loop with readaheads (assuming you know your access patterns) and then RWF_NOWAIT and fall back to a thread pool when that fails. Of course that only helps if you're reading from a file into a userspace buffer. If you want to sendfile that doesn't help.
There are patches for FreeBSD and Linux sendfile respectively to perform TLS symmetric encryption inside the kernel and use out of band signaling for the key exchange.
