X-Plane's water reflections are a two-step process:
- A (simplified) version of the world is drawn into a texture .
- That texture is used (and perturbed) to draw the surface of the water.
Of these factors, the biggest impact on frame-rate is almost always simplification of the world as drawn into the water texture. At its most detailed ("complete" reflections) that rendering is almost identical to the main sim rendering, and thus reflective water should cut your framerate in half!
In particular, the big hit of drawing the world a second time is that it means drawing objects a second time. Thus the higher the reflection setting, the more objects are being drawn. In a scenario where objects are already the main drag on frame-rate, using complete water reflections will make this even worse.
There are two things to conclude from this:
- A bigger, faster CPU won't let you turn up the water reflection detail. Most decent GPUs handle the shader-side just fine. It's the objects that are the problem.
- You shouldn't turn up the water reflection all the way unless you're taking screenshots. It just doesn't add much visual improvement for the fps hit.*