Let’s say that a given ACME session consists of 4 HTTP transactions, which consists of some delays to allow the server to do DCV.
Compare that scenario to a persistent connection, where the issued cert can be sent immediately to the client. The client would then close the connection. This “happy path” scenario is a clear win since there’s only 1 handshake, and the client can get the DCV-OK notice and certificate without polling.
There is obvious potential for abuse, but as with HTTP keep-alive, a simple timeout should solve this.