Possibly. There's an implementation based on TypedArrays. I haven't benchmarked it against JSON. It's very possible that JSON would be faster in a browser simply by virtue of it being built-in. But you might choose to use Cap'n Proto in the browser anyway if your server infrastructure were based on it. It's nice to have the same format all the way down the stack. And there's generally plenty of CPU to spare on the client side.

All that said, I think Cap'n Proto in the browser will only really start making sense once there is a pure-Javascript RPC implementation over WebSocket.