To provide a good User Experience (UX), mobile app performance must be well tuned. Loads of work is invested into designing screens to provide fast feedback to end user and make third-party service communication as invisible as possible. There is unfortunately a wall that mobile devs cannot go through, making faster external calls especially in difficult environment like poor connection quality. But we back-end developers can minimize that pain, but only as a result of tight cooperation. This presentation will contain a ton of knowledge about designing and developing mobile APIs and experience from the trenches.