Query Monitor is a free WordPress plugin by John Blackbourn. It adds a panel to the admin toolbar that shows what one page load did: database queries, hooks, HTTP calls, PHP errors and the template that rendered the page. On a WooCommerce store it answers the usual question, "which plugin makes the product page slow", with numbers instead of guesses.
Install it on staging first
Query Monitor shows its panel to administrators only, so customers on a live store never see it. It still does its work on every request you make while logged in, which is why staging is the better home for it. Install it from the plugin directory and activate it. The toolbar then shows four figures for the current page: generation time, peak memory, database query time and the number of queries.
Read the queries by component
Open the panel and go to Queries by Component. Every query is attributed to the plugin, theme or core file that made it, with a count and a total time. Sorting by time usually points at one or two plugins.
Duplicate Queries is the next view to open. The same query repeated dozens of times in one request often comes from a loop that reads product meta one product at a time.
Slow Queries lists every query that took longer than 0.05 seconds. That threshold is the QM_DB_EXPENSIVE constant, and you can lower it in wp-config.php when the whole page is slow but no single query stands out.
Check outgoing HTTP calls
The HTTP API Calls panel lists the requests a page sent to other servers while it was being built. Licence checks, currency rates and shipping quotes are the usual suspects. A plugin that calls an external API on every product page adds that server's response time to yours. For each call the panel shows the URL, the response code and how long it took.
Time your own code
Query Monitor also has a small profiling API. Wrap a block of code in two actions and it shows up in the Timings panel:
do_action( 'qm/start', 'shipping_estimates' );
// the code you want to measure
do_action( 'qm/stop', 'shipping_estimates' );
For a value you want to inspect, do_action( 'qm/debug', $variable ); sends it to the Logs panel instead of the page output.
Look at a page as a guest
Some slowness only appears for logged-out visitors, for example when a cache plugin treats them differently. The Query Monitor settings can set an authentication cookie, and with it the panel keeps working in the same browser after you log out.
When you are done
Deactivate the plugin once the slow part is found. It is light, but there is no reason to keep it running on a store that nobody is debugging.