Parallel connections performance issue with http-proxy-middleware express

Question

I have setup a proxy server to an exist website with http-proxy-middleware, but the server is really slow, checkout the waterfall with Chrome development tools, a lot of requests have Stalled status.

waterfall

I try to increase maxSockets with https.Agent, the behavior is the same. The code like blow:

const express = require('express');
const proxy = require('http-proxy-middleware');
const app = express();
const https = require('https');

const originDomain = 'example.com';
const appPort = 8080;

app.use('/',
  proxy({
    target: 'https://' + originDomain + '/',
    secure: false,
    changeOrigin: true,
    logLevel: 'debug',
    agent: new https.Agent({
      maxSockets: 100
    })
  })
);

app.listen(appPort, () => {
  console.log('Proxy listening on port ' + appPort);
});