If you disable
ray traced reflections in the Reflective Shader, you get specular highlights which take very little time to compute. Even so, it still "cheats" overall reflectivity of the rest of the environment by using a fast lookup of the environment stored in the GI cache, so you can't render just highlights without any other reflectivity, but to be frank I woudn't like to let you do that anyway

Disabling ray traced reflections gives you fast specular highlights and a fast approximation to the general ambiance of other reflections, depending on the GI settings.
The reflective (specular) settings in the Default Shader do the same thing as the Reflective Shader with ray traced reflections disabled.
Matt