I agree here. I have had an experience of scaling WebSockets server to 20M connections on a single server (with this one https://github.com/ITpC/LAppS.git). However there are several issues with scaling WebSockets, on the backends as well: mutex locking, non-parallel XOR of input stream, utf8 validation. I do not know the state of the above repository code, it seems that it was never updated for at least 5 years. There were bugs in HTTP parsing in the client part for some cases. Though vertical scalability was excellent. Sad this thing never reached production state.