It uses a caching server that indexes the top 150 relays. So it's reading from the NostrGram cache rather than a dozen or more relays that you have configured. It also has indexes in place that make it much faster to locate specific events. When you react to a post or Zap or write a note, that's posted to all your configured relays in the usual way. It's on my to-do list to automatically index any relays the user has configured that aren't in the top 150, but that's not in place yet.