On the Symfony Finland site they've posted a retrospective about moving an application from Symfony 3 to Flex including the work that was done in the switch and the performance of the result.

Earlier this year I did some experimenting with a Hybrid state object between Symfony Twig templates and front end JavaScript frameworks. Since that time I did that experiment, the Symfony Flex project has progressed. I thought I would try how to port the state prototype to Symfony Flex.

[...] In my case the application was rather simple and all built in the AppBundle, which is best-practise in Symfony3 for many applications. I mostly had to move files and configurations around and change namespaces.

He starts by spending some time talking about the difference between a Symfony 3 environment and the environment Flex provides. He then goes through the eight or so steps to move from one to the other including file/directory changes and configuration updates. Next comes the look at performance differences between the two. Unsurprisingly Flex came out on top in every measurement he threw at it.

In addition to the new structure, the apparent improvement in performance is obviously welcome. This would likely be even more evident where I could leave more dependencies out, for example in API workloads. This obviously won't magically push Symfony/PHP into Golang or Node.js territory for raw API throughput, but for existing large code bases it could provide a low-effort boost.