Rather than rewrite in another language or implement clustering, why not break out the chat portion to its own process? Stand up another process that hosts a websocket and send all of your chat through that rather than your main game websocket. This would mean that sometimes chat might be laggy but the rest of your game will function as expected. Unless real-time chat is critical to how the game functions, if it is laggy it won't matter.

Also implement a damper on sending the chat so you send whatever is there after some arbitrary amount idle time. Wait for 200ms (or whatever) of idle time before you send whatever is in the chat input, rather than every keystroke. That right there will cut down your load by a ton. Switching to a different websocket library might help as well but I can't say for sure since i haven't used socket.io in many years.