Why are radiosity techniques computationally expensive?

Radiosity techniques are computationally expensive because they move beyond simple direct lighting to model light as energy interacting across an entire scene.12

Unlike techniques that trace individual light rays, radiosity considers the diffuse reflection of light between all elements in a scene. This means that every object's appearance is influenced by the light reflected from every other object—for instance, a brightly colored surface will cast a subtle glow of its own color onto surrounding surfaces. Because the algorithm must calculate these complex, global inter-reflections for all elements to achieve high levels of physical realism, it requires significant processing power, similar to the intensive demands of ray tracing.12