On the Symfony blog they have a post covering the result of some changes to how the framework handles incoming requests in Symfony 4: a performance and speed increase in the router making it even better than before.

Symfony 4 is the fastest PHP framework according to independent benchmarks, but we are continuously working on making it faster. In Symfony 4.1, we improved the Routing component to make it much faster when matching incoming URLs.

The post starts with a look at the two functional pieces of route handling: the generation of a URL to match and the matching the framework performs. Symfony 4 has sped things up by creating a "matcher" class during the compilation phase using some of these suggestions. The biggest change was to modify the regular expression URL matching to combine all patterns into one, reducing the number of calls to preg_match and locate the correlating route. The new routing requires no changes in your current Symfony 4 application, it just makes all of the updates it needs behind the scenes during compilation.