on nodejs, how do I debug long content download time for tiny payload?

I'm running an nodejs express server on amazon EC2. I wrote a simple speed test route that returns a 146 byte response. Looking at Chrome's network tab, I'm consistently seeing a TTFB around 80 - 120 ms, which I understand to be pretty reasonable. But I'm also consistently seeing a Content Download time of 100+ ms, which seems really long since my response is just the word "Done!" with some basic http headers. I'm using http, not https, and I created a separate EC2 instance for my testing that has no outside load on it. Is that download time reasonable, and if not, how do I debug it?

My code looks like:

app = express();

app.use(function(req, res, next) {
  if (req.url === '/speed_test') {
    res.write('Done!');
    return res.end();
  } else {
    return next();
  }
});
... the rest of my app ...