The one thing that you have the most control over in your project is the content of your scenes. And sometimes, it's these very same intentionally creative decisions that have the greatest negative impacts on performance. 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 thinking and experimentation, you can achieve (nearly) identical visual results with much better performance.
"Quality" is not only how it looks, but also how it feels. Optimizing for user experience, including high frame rates and low latency, is as fundamental a design decision as visual quality.
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...