Last time I checked, Relay.js did not support session-based NetworkLayer (only one NetworkLayer could be used at the same time).
Thus, queue-hack (https://github.com/codefoundries/isomorphic-material-relay-starter-kit/blob/master/webapp/renderOnServer.js#L66) was required to support multiple sessions. It cannot be used in production as each render is completely blocking another render (including data fetching).
What's the current status on this issue? Where can I follow the progress (github issues) and possibly help?
This is the GitHub issue you're looking for, and great progress has been made on making most of Relay "contextual" at this point. See that issue for more details.