It’s hardware-dependent. For PowerVR hardware, and other GPUs that use tile-based rendering, using discard means that the TBR can no longer assume that every fragment drawn will become a pixel. This assumption is important because it allows the TBR to evaluate all the depths first, then only evaluate the fragment shaders for the top-most fragments. A sort of deferred rendering approach, except in hardware.
Note that you would get the same issue from turning on alpha test.