This is reality in rendering.
When you hit an object with an eye ray cast into the scene, you go through the BRDF to determine 3 main components:
Diffuse + Specular (Reflection/Refraction) + emission. The real world has no ambient term. The first thing is that every material has a diffuse term. That is, it scatters light evenly in all directions. A specular term would only apply to surface that bounce light back to your eye within a threshold along the true reflected ray.
1. There are more materials that scatter light evenly than materials that scatter light in a concentrated cone (i.e. specular).
2. When evaluating a diffuse + specular term, you have to not only compute rays towards a light source for direct lighting but also rays that have bounced light onto your object as well (this is global illumination).
3. The specular term (i.e. reflections) aren't on most objects. In real life we don't see mirrored surfaces all over the place. These surfaces are like gold, copper, plastic, etc.. But the mirror surfaces ALSO have diffuse on them.
In short, computing GI rays will apply everywhere on every object. Reflection rays are only on surfaces that show reflection. By deduction, it proves that GI is more expensive than reflections. NOTE: this has nothing to do with getting rid of noise in reflections (which adds another complexity). Since a game will have far fewer reflective surfaces, GI will be more expensive assuming the number of rays cast are equal.
Mirror reflections are very easy because you don't need to blur them. It will require only 1 ray to get a perfect mirror. This is not reality except for perfect mirrors. Not metals.
That is correct. IF and only IF you go for pure GI. If you go for ambient occlusion only, you can get away with not having to evaluate a shader on another object when you determine that it's occluding your material. AO only needs a scalar value. What's the average occlusion value given a set of rays cast around a hemisphere. In GI, you need to ask the question, not only how many objects are occluding but what is their average color. This requires evaluating another shader on another object.
GI is more expensive.