Overdraw
When the same pixel is shaded more than once in a frame because several surfaces cover it — wasted work for the GPU.
Draw the triangles in arbitrary order and a pixel may take color from a wall, then be overwritten by a nearer pillar, then again by an object in front. Every overwrite ran a fragment shader for nothing. The problem eats fill rate and hits hardest on mobile tile-based GPUs and on transparent layers, which can't be depth-rejected.
The remedies are sorting opaque geometry front to back so the z-buffer discards hidden pixels early, or running a z-prepass. Profilers show overdraw as a heat map where red areas were drawn over many times.