This plugin measures the time it takes your checkout to load using hooks. All events are logged to files via the WooCommerce Logger Class. View them via WooCommerce >> Status >> Logs. 
The WooCommerce Block Checkout is not yet supported. You must use the classic shortcode based checkout for now.
<h3>Feature list</h3>
<ul>
<li>Timer function for WooCommerce Shortcode Checkout aka Classic Checkout.</li>
</ul>
<h3>Roadmap</h3>
<ul>
<li>Timer function for WooCommerce Blocks Checkout</li>
</ul>
<h3>Documentation and Support</h3>
More documentation at my website. https://conschneider.de/checkout-profiler-for-woocommerce/

Checkout Profiler for WooCommerce

Wondering whether your WooCommerce checkout is slow? This plugin will tell you.

Forge12 Interactive GmbH

F12 Profiler analyzes your WordPress site comprehensively and delivers concrete, prioritized recommendations to improve load times. The plugin combines server diagnostics, asset analysis, database optimization, and plugin impact assessment in a modern, clean interface.
<h4>Features</h4>
Analysis & Diagnostics
<ul>
<li>Server analysis (PHP, MySQL, OPcache, web server, SSL, HTTP/2, compression)</li>
<li>Automatic site crawler with load time measurement (desktop & mobile)</li>
<li>Asset analysis (CSS, JS, images) with optimization potential</li>
<li>OPcache status & configuration recommendations</li>
<li>PHP profiler (runtime, queries, hooks, plugin ranking, culprit analysis)</li>
<li>Plugin impact analysis (assets, hooks, queries per plugin)</li>
<li>PageSpeed Insights integration (Lighthouse Scores + Core Web Vitals + CrUX)</li>
<li>Performance score with 6 weighted categories (A-F grades, trend delta)</li>
<li>Recommendation engine with automatic optimization suggestions</li>
</ul>
Optimization Tools
<ul>
<li>Lazy Loading (images and iframes)</li>
<li>Font Display Optimization (font-display: swap/optional/fallback/block)</li>
<li>Script Defer Manager (automatic defer for scripts with exclusion list)</li>
<li>Auto Image Dimensions (prevents Layout Shifts / CLS)</li>
<li>HTML Minification (whitespace, comments, empty attributes)</li>
<li>CSS/JS Minification (file-based cache)</li>
<li>WordPress Bloat Removal (emojis, embeds, Dashicons, jQuery Migrate, XML-RPC, Heartbeat, and more)</li>
<li>Browser Cache (.htaccess-based)</li>
<li>DNS Prefetch & Preconnect (domain detection from asset scan)</li>
<li>Database Analysis (8 categories with detail view)</li>
<li>Autoload Manager (granular wp_options management)</li>
<li>Background Asset Scanner with conflict detection</li>
<li>File-based Page Cache with TTL and URL exclusion (Pro)</li>
<li>CSS/JS Asset Manager – disable, defer, async per page or globally (Pro)</li>
<li>Conditional Loading – shortcode, block, regex, archive, login status, post type, plugin (Pro)</li>
<li>Smart auto-suggestions with source-based plugin detection after full scan (Pro)</li>
<li>Asset Preload Manager – Link headers for critical assets (Pro)</li>
<li>Critical CSS Generator & Applier – above-the-fold CSS per template (Pro)</li>
<li>Unused CSS Removal – per page template, safelist, regex safelist (Pro)</li>
<li>WebP Image Conversion with batch queue and backup (Pro)</li>
<li>LQIP Image Placeholders – 20px blur-up previews with IntersectionObserver (Pro)</li>
<li>Preload Key Requests – LCP images, fonts, critical resources (Pro)</li>
<li>Script Delay – delays third-party JS until first user interaction (Pro)</li>
<li>Local Google Fonts – GDPR-compliant WOFF2 self-hosting (Pro)</li>
<li>YouTube/Vimeo Facade – thumbnail + play button instead of iframe (Pro)</li>
<li>Database Cleanup – one-click per category or bulk action (Pro)</li>
<li>Redis Object Cache Management – drop-in installation/removal (Pro)</li>
<li>Scheduled Crawls & Cleanups via WP-Cron (Pro)</li>
<li>Performance Budgets with email/webhook notifications (Pro)</li>
<li>Settings Import/Export as JSON (Pro)</li>
<li>One-Click Optimizer – automated optimization with backup & restore (Pro)</li>
<li>License Management with activation/deactivation (Pro)</li>
</ul>
User Interface
<ul>
<li>Modern iOS-style design system</li>
<li>React 18 + TypeScript SPA</li>
<li>30+ pages/tools with dedicated configuration</li>
<li>Context-sensitive info sections on all pages</li>
<li>Pro features visible with lock icon and upgrade prompt</li>
</ul>
<a href="https://www.forge12.com/f12-profiler-pro/" rel="nofollow ugc">Get F12 Profiler Pro</a>
<h4>System Requirements</h4>
<ul>
<li>WordPress 6.0 or higher</li>
<li>PHP 8.0 or higher</li>
<li>MySQL 5.7 / MariaDB 10.3 or higher</li>
</ul>

F12 Profiler

Excellent! After P3 doesn’t work with PHP7+ anymore, F12 seems finally the one stepping in to fill that gap.

Update [18-Jun-19]. After some small contributions of mine to the dev efforts, the plugin developer has decided to add me to the team, long after this review was written.

Finally an alternative to P3

Comprehensive WordPress performance analysis with crawling, load time measurement, server diagnostics, and integrated optimization tools. Free.

Checkout Profiler for WooCommerce vs F12 Profiler