A modern programming language that makes developers happier.

kotlinlang

Is there any performance benefit (or even penalty) in making all classes of an application (not library) `internal`?
Like in Swift where when everything is internal and thus never used from other modules it can be optimized and inlined quite a lot.

Probably nothing will change. Except longer, a bit mangled names.

On the JVM, nothing will change apart from longer, mangled names as Mike said, which takes a bit more space, although it's close to insignificant.