It makes absolutely no performance difference at runtime, and no observable difference at compile time. Where it matters is not performance, but in the understandability and maintainability of the code. If you use very specific imports, or so the theory goes, then the person reading your code can very quickly understand where each class that's referred to comes from.
Personally, I find this rationale very weak in these days of powerful IDEs. If you're reading code in an
IDE, it can tell you instantly what package any given class belongs to in an instant.
Another slightly more defensible position says that using very specific imports makes it clear what dependencies a class has. Again, tools can tell you this same information, but the imports do give you a good idea, at a glance.