i am working in 2 different projects using symfony 2.8.12 and having the same problem: performance.
It takes too long to load ( not all the times, but the most part ) and when i look the profiler there is always one "guilty component": it can be the firewall, the controller, the profile listener, the kernel response....being the execution time of several seconds ( sometimes longer than 10 ).
Reading in different threads i tried to set the db as a fix ip ( in case it is a dns lookup problem ), modified some parameters in php.ini but nothing changed. This is hapenning both in my local and in the remote environment where it has even PHP acceleration and OPCache enabled.
I am not doing nothing special on my code, even i get long times in "hello world" pages, it's a little frustrating :)