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.

wpaluchmits

MITS Profiler is a tool by <a href="https://mits.pl" rel="nofollow ugc">MITS</a> — a WordPress and WooCommerce agency based in Poland. It adds a diagnostic bar to any page by appending <code>?mits_prof=1</code> to the URL. It captures timing data from the moment PHP starts and presents it in a dark-themed panel at the bottom of the page — visible only to logged-in administrators.
Tabs included:
<ul>
<li>Timeline — absolute and relative timing between WordPress lifecycle hooks (plugins_loaded → init → wp_head → wp_footer)</li>
<li>Hooks — top 50 slowest action/filter hooks with total time and call count</li>
<li>Options — most-queried WordPress options + full autoload audit with sizes</li>
<li>Plugins — loaded PHP file sizes per plugin (proxy for load time)</li>
<li>Assets — all enqueued scripts and styles, external ones highlighted</li>
<li>HTTP — outgoing wp_remote_* requests with response time and status</li>
<li>SQL — all database queries with slow-query highlighting (requires SAVEQUERIES)</li>
<li>PHP — PHP version, OPcache status and hit rate, memory and execution limits</li>
</ul>
Key features:
<ul>
<li>Zero HTTP loopback — works behind Cloudflare, reverse proxies and strict firewall rules</li>
<li>No external dependencies or requests</li>
<li>Admin-only: the panel and all data are never shown to guests</li>
<li>Export to JSON or CSV for offline analysis</li>
<li>Works on front-end pages, WooCommerce pages, and WP Admin</li>
</ul>
SQL query logging requires adding the following to <code>wp-config.php</code>:
<pre><code>define('SAVEQUERIES', true);
</code></pre>

MITS Profiler

Lightweight WordPress performance profiler. No HTTP loopback requests — works behind Cloudflare and any proxy.

Checkout Profiler for WooCommerce vs MITS Profiler