In a quick post to his site Fabien Potencier (of Sensio Labs and the Symfony framework) has a post sharing some information around the performance of Symfony 4 and what kind of speed you can expect in your applications based on it.
Performance is an interesting and sensitive topic. Suffice to say that most projects should not care too much; modern PHP frameworks are fast enough for most use cases and projects. And PHP 7 performance improvements help a lot as well. But people like to compare frameworks, and I guess performance is one way to do so.
What about Symfony 4? During its development cycle, we did some nice performance optimizations: mainly to improve the router and container speed. But several non-related changes in Symfony 4 also help with performance... Your mileage may vary depending on your specific project of course.
He talks about some of the testing and evaluation that's already been done on "Hello world" script performance but points out that this isn't a very real world situation. He shares these metrics, however, to at least give a baseline to compare it against previous Symfony versions. The post also links to another resource that shares these and other benchmarks related to various PHP projects.