Why wait about 20 sec the child process before send stream end event to main process?

Question

I try read records from db and send back to client as ajax request's response. Here my test program:

Child process(test.js):

const zlib = require('zlib');
const t = require('through2');
const QueryStream = require('pg-query-stream');
const JSONStream = require('JSONStream');
const {
    Pool
} = require('pg');
const config = require('./modules/config');
const pool = new Pool(config.get('db'));
 (async function () {
    const conn = await pool.connect();
    var query = new QueryStream('SELECT * FROM teszt001.records ORDER BY rec_time ASC LIMIT 500', []);
    let first = true;
    var stream = conn.query(query);
    stream.on('end', () => {
      conn.release();
    });
    stream.pipe(t.obj(
      function (chunk, enc, cb) {
        if (first) {
          first = false;
          this.push(Object.keys(chunk));
          }
        this.push(Object.values(chunk));
          cb();
        }
   ))
 .pipe(JSONStream.stringify()).pipe(zlib.createGzip()).pipe(process.stdout);
})();

Main process:

const express = require('express');
const app = express();
const {
  fork
} = require('child_process');
const common = require('./modules/common');

app.get('/', (req, res) => {
  let view;
   if (common.identity(req).level == '1' || common.identity(req).level == '2') {
   view = 'adm';
  } else {
  view = common.identity(req).username;
 }
 res.set('Content-Encoding', 'gzip');
 res.set('Content-Type', 'text/plain');
 let queryFilter = {};
 let child = fork('./test.js', [req.session.user.schema, JSON.stringify(queryFilter), view], {
  stdio: ['ignore', 'pipe', 'inherit', 'ipc']
 });
 child.stdout.pipe(res);
});

server.listen(3005, () => {
  console.log(`Server started!`);
 });
module.exports = server;

When all the data goes through to the main process, the end event takes about 20 seconds. I did some wrong or is it a bug?