longtasks_via_observer.js

const puppeteer = require('puppeteer');
const devices = require('puppeteer/DeviceDescriptors');

const Good3G = {
    'offline': false,
    'downloadThroughput': 1.5 * 1024 * 1024 / 8,
    'uploadThroughput': 750 * 1024 / 8,
    'latency': 40
  };
  
const phone = devices['Nexus 5X'];



async function run(url) {
	// I can't get the experimental flag working on chromium, so let's start system chrome
    const browser = await puppeteer.launch({headless: true});	
    let page = await browser.newPage();
    const client = await page.target().createCDPSession();
    await client.send('Network.enable');
    await client.send('Network.emulateNetworkConditions', Good3G);
    await client.send('Emulation.setCPUThrottlingRate', { rate: 4 });
	await page.emulate(phone);
	
	// register observer
	await page.evaluateOnNewDocument(async function() {
		window.longtasks = [];
		const observer = new PerformanceObserver((list) => {
		  for (const entry of list.getEntries()) {
		    // `entry` is a PerformanceEntry instance.
			  longtasks.push([entry.attribution[0].containerSrc, entry.attribution[0].containerType, entry.attribution[0].name]);
		    /*console.log(entry.entryType);
		    console.log(entry.startTime); // DOMHighResTimeStamp
		    console.log(entry.duration); // DOMHighResTimeStamp*/
		  }
		});

		// Start observing the entry types you care about.
		observer.observe({entryTypes: ['longtask']});
	});
	
	// load page
	await page.goto(url, {"timeout": "120000"});
	// get results
	let results = await page.evaluate(function() {return window.longtasks});
	console.log(JSON.stringify(results, null, 2));
		
    await page.close();
    await browser.close();
}

run("https://www.stern.de");