Doesn't sound so impressive. I've done close to a million on a single Digital Ocean droplet using nchan[0] before. Latency was reasonable even with that many connections, you just need to set your buffer sizes carefully. Handshakes are also expensive, so it's useful to to be able to control the client and build in some smart reconnect/back-off logic.

[0] https://www.nginx.com/resources/wiki/modules/Nchan/