On the Symfony Finland site there's a new post sharing the results of some recent benchmarks of the differences between running the framework on PHP 7.1 and PHP 7.2:

PHP 7.2 will be launching soon, in fact, it has already reached Release Candidate status. I was exploring Symfony Flex with Docker setup and thought I would do a quick round of tests to compare the differences in PHP 7.1 and 7.2 (RC4) regarding performance with a few benchmarks.

[...] The benchmarked application is the Symfony Flex port of the hybrid application I did back in January. The project now has the required configuration to run it with Docker, and you can find the full source on GitHub.

The post then shares some of the results and conclusions of the test runs, showing the differences between the two versions. In one set of tests, they're calling the front page controller with Twig rendering and in the other a backend controller without the display rendering. PHP 7.2 ends up performing slightly better than PHP 7.1 overall but not by very much in most cases. The more dramatic change is on the backend, though, with a good jump in performance for a Symfony Flex application.