How to speed up the class scan via Reflection API in Java?

I use org.reflections library to scan ClassPath and get classes. Here is my code:

Reflections ref = new Reflections();
Set<Class<? extends Service>> classes = new HashSet<>();
for (Class<? extends Service> subType : ref.getSubTypesOf(Service.class)) {
    if (!Modifier.isAbstract(subType.getModifiers())) {
        classes.add(subType);
    }
}

But I faced a problem. It takes too much time. At the same time I can't set a package

new Reflections("my.pack");

because I want to save an ability to add Service classes in the future. Hot can I accelerate this process? Is it possible to exclude rt.jar packs?