In order to be capable of restricting her target to the ground, Light Binding would by necessity need to have a sizeable amount of mass.
The amount of mass in a stream of photons (such as a sunbeam) is too small to physically impede a solid body of mass such as a person. Lux's Light Binding, however, is capable of binding even a sizeable bulk of mass like a 6-stack Cho'Gath.
In order to perform its binding function, Light Binding must, by necessity, be a hard-light projection. Because it needs to have sufficient mass to bind and restrict targets in a wide variety of sizes and strengths, it is subject to gravitational and aerodynamic impedance.
Hence, it has a travel time visible to the naked eye, even though it's constructed of photons (light).