Brett

Frontend Features: 
 – adds a toolbar to the bottom of WordPress pages that will show meaningful information that helps developers to create optimized WordPress sites. 
 – Info includes SQL queries, query vars, peak memory usage, and a filetrace.
Backend Features: 
 – Provides a very basic checklist with suggestions for development. 
 – Shows basic server stats and information 
 – phpInfo 
 – WP rewrite rules (coming soon)

Developer Toolbar

Adds easy to access data for developers, including a fixed toolbar to the bottom of pages that shows query data and memory usage among other data.

Forge12 Interactive GmbH

This plugin will track the load time of each plugin, javascript and css file 
to help you optimizing the performance of your WordPress site. It will help you 
to figure out which plugin / resource causing the slowness of your page.
Track time for:
<ol>
<li>Core</li>
<li>Theme </li>
<li>Plugins</li>
<li>Resources (JavaScript, CSS)</li>
<li>External files</li>
</ol>

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

Trace the execution time of plugins, themes and core files to find the bottlenecks of your system.

Developer Toolbar vs F12-Profiler