Oh, no wonder… Thanks for looking into this.
I didn’t really make the level. It’s from a mod for Quake 1 called Arcane Dimensions. I simply brought it into Blender, and baked textures on a continuous copy of the mesh, with added lightmap UVs.
I’ll try using ~16 pcs instead of one and see how that goes.
The reason I wanted to do it as one piece is to A. spare myself from lengthy material setup (original level had ~50 pieces and 50 materials), B. reduce number of draw calls to an absolute minimum (since total soft budget is 60 draw calls in the view).