GenTime, measures the generation time of each page

This is one of the support plugins I usually install when I do optimisation tasks.

It's light, simple and useful. It only does one thing. It doesn't have any settings or configuration. Install, activate and you're done.

GenTime inspects the time it takes to generate each page you visit and displays it in the top bar of the WordPress administration. This is especially useful for getting a pretty rough estimate of which pages take the longest to load when you need to attack specific problems in specific places.

Keep in mind that these values are always approximate because they are subject to possible intermittent server responses. So take them for what they are, a reference.

Its author is Sybre Waaijer, a developer who is also the founder of another popular plugin, The SEO Framework, the most lightweight SEO plugin for WordPress today.

Sybre says that if the plugin shows 0.070s or more on a clean WordPress installation, you should consider switching hosts.

He also notes that this is true if you are using a "classic" theme. If your template is FSE (Full Site Edition), the limit would be 0.140s (as for example with Twenty Twenty-Four as a template, because FSE is twice as slow).

There are two filters you can add to your functions.php, one is to change the minimum user role that will be able to see the GenTime measurement:

add_filter( 'gentime_minimum_role', 'my_gentime_minimum_role' );
function my_gentime_minimum_role( $default = 'install_plugins' ) {

    // See https://codex.wordpress.org/Roles_and_Capabilities for a list of role names
    $role = 'edit_pages';

    return $role;
}

And another to change the number of decimals to display:

add_filter( 'gentime_decimals', 'my_gentime_decimals' );
function my_gentime_decimals( $default = 3 ) {
    return 4;
}