A drag is not a very good way to think of it-- drag is a force that can achieve a slowed speed if there is always some force pushing it forward, like a motorboat engine. In fact, if the idea is to slow light down to c from some other faster speed, then what was that other speed? It doesn't answer anything to imagine it that way. I would just say, either the speed of light would be infinite, or it wouldn't. If the former, what a strange world that would be, and if the latter, then why not c? For the reasons mentioned above, particles with zero rest mass define a speed limit-- and it wouldn't make much sense for that speed to be infinite. Basically, the spirit of relativity requires there be a single speed for massless particles, and why that speed turns out to be c is totally unknown and probably always will be.