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.
It is true that RSA operation over IPC is slower than doing it internally. But the latter is by magnitudes faster than the former, therefore the slowdown is negligible in practice.
You can find the numbers in the FAQ section of the linked website.
