Description

The upcoming Symfony 5.0 release gives us once-in-a-two-years opportunity to improve things. I'd like to improve this: events/listeners take too much time in the "dev" environment.

Look at this profiling of the Symfony Demo app: https://blackfire.io/profiles/f35213c7-d5b5-4eb4-8315-4ad3f24ac9e9/graph

Some things that look "wrong":

• Symfony\Component\HttpKernel\Debug\TraceableEventDispatcher::getNotCalledListeners is the slowest part of the application and takes 15% of the total time.
• Symfony\Component\EventDispatcher\Debug\WrappedListener::getWrappedListener is called 5,135 times
• Symfony\Component\EventDispatcher\Debug\TraceableEventDispatcher::getListenerPriority is called 279 times

Can someone please take a look why this is so slow ... and see if we can improve it even if it requires making some minor breaking changes? Thanks!