real-time chat whats the best pub sub for extreme scale

Hey everyone, so we're working on this new real-time chat thing, and we're expecting to handle millions of people chatting at once. We're thinking about using a pub/sub system. Redis Pub/Sub seems easy, but I'm worried it won't scale up enough. Kafka is really powerful, but it might slow things down a bit. What have you all used for super-big, real-time messaging systems, and what were the downsides?