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In this setup you can not use any of the new Symfony and Platform features as the legacy bundle ("bridge") is not used for integrating the two system together. So this is more of a first step toward eZ Platform, getting your legacy code to work with eZ Publish 5.x.

If you choose this because of performance concerns setting up a combined setup, recommendation is to move to PHP 7.x when moving to Platform setup which will result in a overall faster setup then a pure legacy setup on PHP 5.x.
You can either do this on eZ Publish 5.4 which supports PHP 7.0, or even better you can jump directly to eZ Platform v2 with legacy "bridge" in order to get all the newest Platform and Symfony features, and run that on PHP 7.1 or higher.