A store feels slow. Someone has installed forty extensions over three years. The usual advice is "disable them one at a time and see what helps." That is not debugging. That is guessing with extra steps, on production, with a fingers-crossed deploy at the end.
You can do better. Magento 2 makes the cost of an extension measurable if you know where it spends your time. Here is how I find the culprit instead of guessing.
Where an extension actually costs you
A third-party module doesn't just "add features." On every request it can add work in four places:
Plugins (interceptors). Every <plugin> in a module's di.xml wraps a method. Magento compiles these into interceptor classes, and at runtime each one is a layer the call passes through: before, around, after. A module with twenty plugins on hot paths is twenty extra layers on requests that fire constantly.
