symfony/symfony — comment on commit 997f354d53972fdbb327e1a3c94eddf1f9001d2d
File: src/Symfony/Component/ClassLoader/README.md line 32
By stof:

this is not really true. Namespaces will be checked in the order in which they are registered so a most specific namespace should be registered first for performance. But doing the opposite will not break things (it will simply check first in the ORM folder and fail to find the class there before looking in the doctrine-common folder).

Leaving the loop for the first matching namespace was an old implementation