While analyzing Symfony 4 performance (see #24808) I saw that TraceableFirewallListener is super slow. It's about 20 times slower than the average debug listener (~22ms vs. ~1ms):

It's not a Symfony 4 thing. In previous Symfony versions this was called FirewallListener and it was also 20 times slower:

I just open this issue in case someone experienced in the Security bundle can take a look and see if this can be optimized somehow. Thanks!