The speed of light in confined QED vacuum: faster or slower than c?

We consider the propagation of light in the QED vacuum between an unusual pair of parallel plates, namely: a perfectly conducting one (ϵ→∞) and an infinitely permeable one (μ→∞). For weak fields and in the soft photon approximation we show that the speed of light for propagation normal to the plates is smaller than its value in unbounded space (in contrast to the original Scharnhorst effect [K. Scharnhorst, Phys. Lett. B 236 (1990) 354, G. Barton, Phys. Lett. B 237 (1990) 559, G. Barton, K. Scharnhorst, J. Phys. A 26 (1993) 2037]).