Earlier quoted context omitted.
I assume you are referring to Keyless SSL. https://blog.cloudflare.com/keyless-ssl-the-nitty-gritty-tec... For Keyless SSL, it is necessary to make RSA operations asynchronous, since the operations are requested over the TCP network (which may have big delays). OTOH Neverbleed degelates the operations within the same server using Unix sockets. So there is no fear of such delays. And the server spawn a dedicated threa…
The point is that it requires TLS handshakes to be done in a multi-threaded system for a server handling high concurrency. Many servers are multi-threaded, but many are not. Using the proposed technique in a Node.js process, or nginx, is going to severely limit the number of new connections per second.
You seem to have confusion between TLS handshakes and RSA operations.
In OpenSSL (which is used by many servers including node.js, nginx), RSA operation is always synchronous. Therefore, using Neverbleed does not impose new limits regarding concurrency.