Base Pass is responsible for many things - all related to preparing data for next passes after it. So:
- Running the materials and saving their output (color, roughness, metallic etc.) to the GBuffer
- Applying DBuffer decals (so watch out if you have a lot of these)
- Reading lightmaps
- some other small tasks, like fog
Either you have a big number of objects, heavy shaders or many decals. And it’s all “multiplied” by the game resolution.
I explain it in detail in my video at 22:50: UE4 Graphics Profiling: All Categories Guide (Rendering Passes) - YouTube
Hope it helps.
edit: Early Z-Pass probably helped because it allows to discard occluded areas before the Base pass. It has a cost of it’s own, but here I can see it was worth it.
Regards,
Oskar