Speed of light is in general $c/n$ where $n$ is a refractive index. But for example introducing two parallel plates with very small spatial separation will perturb the energy density of vacuum reducing it in between the plates, thus effectively lowering $n$. So the rate of induction in this part of space would increase giving larger than $c$ light speed.
This question has even been publicly studied ( since such research is highly classified) there is not too much to read on the subject and it's hard to find. See https://en.m.wikipedia.org/wiki/Scharnhorst_effect
Basically since Casmir plates lower the density of " Dirac sea particles" photons spend less time interacting with them thus spending less time as subliminal products of those interactions effectively moving at a velocity larger than c
You can find a related question on stackexchange here
Related to the Casimir effect , see this link.
A hand waiving, intuitive answer would be, if you change the average energy density in a region of space, you change the curvature, so you change the geodesics, but this is far from a decent answer.
I don't think there is an easy answer to this question, but I like it. Therefore, I propose to actually perform the experiment. Measure the speed of light in a direction parallel to the Casimir plates, inside a Casimir cavity.