OpenGL Programming Guide: The Official Guide to Learning OpenGL, Version 4.3, Eighth Edition

In-Application Profiling

It is possible for your application to measure its own performance. A naïve approach is to simply measure the amount of time taken for a particular piece of code to execute by reading the system time or just measuring framerate (besides, framerate is a fairly poor measure of application performance). However, assuming your application is efficiently written, the goal is for the graphics processor to be the bottleneck, and therefore you are interested in the amount of time it takes to process parts of your scene.

You can make these measurements yourself. In fact, some tools may use very similar mechanisms to measure GPU performance by injecting equivalent sequences of commands into your application’s rendering thread. ...