@stof If I read this correctly:

+  * The method `createBuilder` was removed from `FormTypeInterface` for performance
 +    reasons. It is now not possible anymore to use custom implementations of
 +    `FormBuilderInterface` for specific form types.

This is not regarding ne MyClassType(), it is regarding custom implementations with createBuilder()
as you can use in the Controller. Or am I wrong?

Or am I wrong?

Yes, you're wrong. The createBuilder method from the controller is in the Controller class, this is about FormTypeInterface::createBuilder, something you could only use in Symfony 2.0. For instance, this method was used in the FormType.

For the reference, this issue is talking about "Allowing "new" tags with the "prototype"" of the /cookbook/form/form_collections article. This should be updated: The example should be changed (you can see an example of that in the first issue mentioned above).

A .. caution:: should also be added in the collection field type reference.

I just wanted to verify if it actually a performance impact as it is described. If it is so it should be addressed.

There is a performance impact. Whe you pass a type instance to the FormBuilder::add() method, it registers it in the form registry, which requires resolving the inheritance tree for this type. If you register it again, it requries resolving again, while Symfony 2.1 optimizes the resolution to be done only when registering the type rather than each time you use it.
for a the inner type of collection field, this has a major impact, as the type will be used for each item of the collection.

Closing it since as of 2.8 / 3.0 types will always be registered by the FQCN.