Node.js pg first query takes longer than subsequent queries

pg-basic.js

'use strict';

const pg = require('pg');
const PG_URL = process.env.PG_URL || 'postgres://postgres:postgres@127.0.0.1/postgres';
const ITERATIONS = Number(process.env.ITERATIONS || 10);
const VARIANT = Number(process.env.VARIANT || 0);

async function benchmarkPg() {
		const client = new pg.Client({
			connectionString: PG_URL
		});
		await client.connect();

		const times = new Array(ITERATIONS);
		let rows;
		for (let i = 0; i < ITERATIONS; i += 1) {
			const timeStart = process.hrtime();
			if ((i + VARIANT) % 2 === 0) {
				rows = (await client.query('SELECT 1')).rows;
			} else {
				rows = (await client.query("SELECT * FROM (VALUES ('val1', 'val2')) t")).rows;
			}
			const timeEnd = process.hrtime(timeStart);
			times[i] = timeEnd;
		}

		console.log('ran %d queries; times:', ITERATIONS);
		for (let i = 0; i < ITERATIONS; i += 1) {
			const timeEnd = times[i];
			console.log('%d: %f ms', i, (timeEnd[0] * 1000 + timeEnd[1]/1000000));
		}
}

benchmarkPg();

My results show that the first query indeed takes longer:

ran 10 queries; times:
0: 2.069616 ms
1: 0.542416 ms
2: 0.382777 ms
3: 0.415688 ms
4: 0.371435 ms
5: 0.361968 ms
6: 0.321762 ms
7: 0.302884 ms
8: 0.275773 ms
9: 0.219883 ms

Adding this line right after .connect() changes nothing, which indicates it's a connection initialization or query initialization effect:

await new Promise(function(resolve) { setTimeout(resolve, 500); });

(the .connect is not resolving prematurely, that is)