Some decisions that impact performance the most are your intentionally creative ones. Maybe you want hyper-realistic graphics with high-fidelity sound because its gotta be so awesome! Realizing you must dial that down may constitute a difficult design compromise. However, it's also likely that with a little creative outside-the-box thinking and experimentation, you can achieve (nearly) identical visual results with much better performance. The one thing that you have most control over in your project is the content of your scenes.
Quality is not only how it looks, but also how it feels. Optimizing for user experience is as fundamental a design decision as any.
In general, try to minimize the number of vertices and faces in your model's meshes. Avoid complex meshes. Remove faces that will never be seen, such as the ones inside of solid objects. Clean...