The Web performance snippet I use to populate how quick a page loads on my blog, now fixed the iOS 0 milliseconds issue.

webperf.js

(function() {
	/* check to see if the browser is modern enough to support the
	PerformanceNavigationTiming API */
	if (!window.performance?.getEntriesByType) {
		console.warn('Performance API not supported');
		return;
	}

	function showLoadTime() {
		const entries = performance.getEntriesByType('navigation');
		const nav = entries[0];
		if (!nav) return;

		// duration = loadEventEnd - startTime; on iOS/Safari it can be 0 if read too early
		// (PerformanceObserver can fire before load event). Fallbacks for when duration or
		// loadEventEnd is 0 (e.g. WebKit timing quirks, bfcache).
		let ms = nav.duration;
		if (!ms && nav.loadEventEnd > 0) {
			ms = nav.loadEventEnd - nav.startTime;
		}
		if (!ms && nav.domContentLoadedEventEnd > 0) {
			ms = nav.domContentLoadedEventEnd - nav.startTime;
		}
		if (typeof ms !== 'number' || !(ms >= 0)) return;

		const pageLoadTime = Math.floor(ms);

		let loadTimeElement = document.getElementById('pl');
		if (!loadTimeElement) {
			loadTimeElement = document.createElement('p');
			loadTimeElement.id = 'pl';
			const footer = document.querySelector('footer');
			if (footer) {
				footer.insertBefore(loadTimeElement, footer.firstChild);
			}
		}
		if (loadTimeElement) {
			loadTimeElement.textContent = `This page loaded in: ${pageLoadTime} milliseconds.`;
		}
	}

	// Read timing only after load event so loadEventEnd is set (fixes 0ms on iOS Safari).
	function runAfterLoad() {
		setTimeout(showLoadTime, 0);
	}

	if (document.readyState === 'complete') {
		runAfterLoad();
	} else {
		window.addEventListener('load', runAfterLoad);
	}
})();