I can tell you how that works: when the comment is created, an internal copy of that data is displayed internally, at the same time a successful response ia received from the blockchain. So while it seems immediate to the poster, it does take 3 seconds for everyone else.
Now for messaging it’s different, because it really does take 3 seconds for the message to be sent. I don’t know that that really is a huge difference, but if so, it could be handled by a separate off-chain channel that sends the message rapidly, and then the on-chain is basically the persistence.
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