We consider between 25-30 fps the ideal range to shoot for. Highly detailed add ons (scenery, aircraft, etc) can greatly affect frame rate, as can clouds and AI aircraft. You can be bottlenecked on either the CPU or GPU, and depending on which one is your limiting factor, that affects how high certain sliders can be set. Some are cumulative, such as the world objects slider + basically everything else. The more objects you have, the more work the sim does to draw them with detail, shadows, reflections, etc.
Recommended steps for methodically figuring out the best slider settings for your system can be read here or watched here.