Performance is part of game feel. A beautiful game that stutters during an important moment will be remembered for the stutter, not the beautiful lighting.
Start with the largest files. Textures, audio, and data packages usually offer the biggest opportunities for improvement, especially when they are loaded before the player needs them.
Next, watch the frame rate while the game is doing its busiest work. Effects, physics, and too many visible objects can all compete for time in the render loop.
The goal is not to chase a number in isolation. It is to make the experience consistent across the devices your audience actually uses.