Out of curiosity, I implemented the same using websockets. You'll obviously need a real implementation of a request-response interface, but this will do for the test:

    // server
    ws.on('message', msg => {
      switch (msg) {
        case 'collection':
          return ws.send([...generate 500 links ...])
        case 'item':
          return ws.send(...generate item...)
      }
    })

    //client
    ws.send('collection');
    ws.onmessage = e => {
      const msg = JSON.parse(e.data)
      switch (msg.type) {
        case 'collection':
          collection.forEach(item => ws.send('item'))
        case 'item':
          items.push(item);
      }
    }

It finishes in 140-150ms, around the same time as the best HTTP version. That's without any caching and the client requesting each of the 500 items, when in fact the server could just push them. I'm surprised WS is not used more often for data exchange.

EDIT: actually ~50ms after removing the console.log() I added for each message...