  We address a dynamical, spherically symmetric background in beyond Horndeski
theory and formulate a set of linear stability conditions for high energy
perturbation modes about an arbitrary solution. In this general setting we
derive speeds of propagation in both radial and angular directions for gravity
waves and compare them with the speed of light in the case of minimally coupled
photon. In particular, we find that the class of beyond Horndeski theories,
which satisfy the equality of gravity waves' speed to the speed of light,
feature gravity waves propagating at superluminal speeds about a time-dependent
inhomogeneous background.
