Consider that a human can probably only keep up with, at most, a couple chat messages per second (and that's generous). But a Durable Object can handle orders of magnitude more than that. So the scalability bottleneck for the chat use case is actually humans, not the limits of a Durable Object.

Many use cases end up being this way.

With that said, if you really wanted to support a chat room that has more traffic than a single object can handle, the way to do it would be to shard the object. E.g. have 10 chat room "shards", have each client connect to all 10, and randomly choose a shard to handle each message sent. Or if it's the number of clients (not the frequency of messages) that is a limiting factor, have each client connect to only one shard, but have the shards rebroadcast to each other. (This is two possible designs, but there are many other options depending on what you're trying to accomplish.)

If you're getting near the limit of what one Durable Object can handle, you probably should introduce such sharding into your app. Once you have sharding, you can scale easily again, by adding shards.

We'll likely add tools so that the system can handle various sharding approaches automatically for you.