Just have a process running an infinite loop paginating with the cursor and storing the list of unique user ids that are online. Then have a separate process that takes that list of ids and obtains the live data of each channel by querying asynchronously the /streams endpoint filtering by user_id in chunks of 100.
Obviously you need to make sure you have the needed quota to perform the asynchronous calls with Helix. Alternatively you can do them with Kraken for now. You should be able to pull the live data for all the channels in just a few seconds.
Also, you can have multiple cursors running in parallel to detect a streamer that goes live as soon as possible.