Below image depicts the focus areas if you are considering Performance Tuning as area of interest.

As a developer, you can only control Platform (by configuring the hosting environment as per application requirement) & Application (by optimizing design & code implementation with due consideration of functional requirements) and these should be the key focus areas. Though today’s hardware & network nodes are scaled enough, System & Network are beyond your control but still pointers can be raised to concerned team with supported stats if Optimization are required in these buckets as well.