Slash GraphQL subscription is queries only, and hence only flow from server to client only.
Updates happen over an immediate HTTP connection, so this would be realtime as well. While it’s a new HTTP request, we should support HTTP/2 with connection multiplexing, so there shouldn’t be a major difference in performance for updates.
The support for HTTP/2 is in progress and should rollout this week.
However, Dgraph internally implements subscriptions by polling for changes, so you might see a delay of upto 1s by default.