Is wireless charging of my Galaxy S6 slower with a Spigen Case on?

Inductive charging uses an electromagnetic field to transfer energy between two objects. This is usually done with a charging station. Energy is sent through an inductive coupling to an electrical device, which can then use that energy to charge batteries or run the device. The further your device(Or the induction receiver behind you're glassback) is from the emitter, the weaker the signal that it recieves.
Furthermore the the material that is used (For the most part) isn't going to affect your charge other than the added distance.