I'v just migrated a project from Symfony 3.4 to 4.2. My test suite executed 5-6x longer. After investigation I discovered that parameter container.dumper.inline_class_loader with value true is the reason.
It's now a part of Flex recipe for FrameworkBundle: https://github.com/symfony/recipes/blob/master/symfony/framework-bundle/4.2/src/Kernel.php#L31
I think that it's worth consider to add information about that to documentation.