Combine sugar, cornstarch, and 2 teaspoons apple pie spice in a small bowl.
Place apples in a large slow cooker. Stir in spice mixture and toss with lemon juice.
Mix cake mix, brown sugar, and remaining apple pie spice together in a medium bowl. Stir in melted butter until thoroughly blended; mixture will be crumbly. Sprinkle over apples in the cooker.
Cover and cook until apples are tender, on Low for 4 hours or on High for 2 hours. Partially uncover the slow cooker and continue to cook until the topping has hardened, about 1 hour more.