I am trying to build a Dapp and I need to send messages between any two users. The messages would be used to reach a sort of private agreement between them. The agreement then could be published on the blockchain. I want to avoid using smart contracts or transactions to reach the agreement as those would cost gas. I was looking into whisper and wondering if it is a good solution. Moreover, how much time would it take to send and receive a message in whisper?
I have read it is not really suited for real time communications, but how fast or slow can it be?