page.goto() already resolves to the main response:
const response = await page.goto('https://example.org/');
console.log(response.headers);
{ date: 'Sun, 29 Oct 2017 13:35:59 GMT', 'content-encoding': 'gzip', 'last-modified': 'Fri, 09 Aug 2013 23:54:35 GMT', server: 'ECS (lga/1318)', etag: '"359670651+gzip"', vary: 'Accept-Encoding', 'x-cache': 'HIT', 'content-type': 'text/html', status: '200', 'cache-control': 'max-age=604800', 'content-length': '606', expires: 'Sun, 05 Nov 2017 13:35:59 GMT' }