Raw ws WebSocket connections cost about 11.4 KB of server memory each, measured with 5,000 simultaneous idle connections on one Node.js process.
Socket.IO connections cost about 23.7 KB each, roughly double ws, from its room, namespace, and acknowledgment bookkeeping.
Server-Sent Events cost 23.4-29.85 KB per connection depending on the client implementation - not meaningfully cheaper than Socket.IO despite SSE's reputation as the lightweight option.