Check performance up to a certain point in the page's load, multiple times

checkPerf.js

/**
 * To use: call the function at the point in the load you want to check performance for.
 * The page will then reload for @var iterations times to provide multiple measures
 */
function checkPerf() {
	var mean;
	var iterations = 10;
	var times = localStorage.getItem( 'perf' ) ? JSON.parse( localStorage.getItem( 'perf' ) ) : [];
	times.push( performance.now() );
	localStorage.setItem( 'perf', JSON.stringify( times ) );

	if ( times.length >= iterations ) {
		mean = times.reduce( (p,c) => p + c, 0 ) / times.length;
		console.log( Math.round( mean ) + ' ms is the mean over ' + times.length + ' iterations', times );
		alert( 'Performance checking done. Check the console for results.' );
		localStorage.removeItem( 'perf' );
		return;
	} else {
		location.reload();
	}
}