Elm’s Http module does not allow that, but it does let you show a progress bar if your backend can predict the size of the response.
To access data from a request while it’s being received, you need to do the request in JavaScript and use ports. Then use or create a JSON parser that works with streaming data.
If the backend can quickly access a node when given an ID, then there’s another solution: make the backend send only the node IDs in the first response, then the frontend can make one request for each node to get the nodes’ data.