Blackfire is a PHP profiling tool and can be used at any step of your application's lifecycle: during development, test, staging and production, to profile, test, debug and optimize its performance.

Blackfire profiles are generated only for requests triggered by a Blackfire user, or by Blackfire when profiling is automated. That is made possible thanks to our runtime instrumentation. As no other requests will be instrumented, Blackfire will add no overhead for your end users, which makes it safe to use in production.

In addition, we recommend reviewing their Magento addon.