When you use encryption, primary load is during initial handshake between client and server. At the time of initial handshake, keys are exchanged. Load of Secure Network Connection SNC using DIAG to HTTPS is different and can’t be compared.
With use of F5 servers, it can be used for software and hardware acceleration of HTTPS and not for DIAG protocol.
You can also refer SNC guide for more information if you have SAP Partner ID.