It can. I have a demo deployed on Heroku for this app (https://github.com/JangoSteve/websockets-demo), and I've seen it handle maybe about 40 or 50 clients at a time on a single dyno before (and each client streaming a bunch of events), even though it's intentionally implemented in the simplest (i.e. inefficient) fashion possible. The request overhead for normal HTTP request/response goes away, since you only need the request headers and handshakes once at the beginning for a persistent connection.

I'm trying to get this demo working on an extremely inexpensive plain VPS (running Debian 7), and I'm getting "WebSocket connection to... failed: Invalid frame header". Is that what you saw when you as a result of hitting the 40-50 client limit? I am getting the error on a first connection, so it's probably something different.

I'm going to answer my own question - I tracked it down to a bug in lattice-core that occurred when it couldn't parse incoming JSON, and for some reason it killed the socket server. I fixed it by rescue the JSON parse and all is good now.

I haven't explored that a lot, but my thought was that the amount of setup/teardown for polling goes away, so it's at least an order of magnitude (guessing) better.

I have written the framework to use a single socket connection for all updateable objects on the page.