Author here.. If I'm understanding you correctly, I believe that's how things are already working. The very first time a client hits a new domain, the SSL handshake initiates the certificate registration with Let's Encrypt (assuming the domain is part of the whitelist of allowed domains). The certificate is registered in the background, while this first request is paused. The SSL handshake is then completed with the first client once the certificate is successfully issued. This does lead to the very first client's request being delayed a few seconds, but this is a one-time delay (per allowed domain).