Why does fps drop in a building?

Question

Unity

Outside, stable 175 frames

When approaching a building, and even more so inside, fps is getting smaller and smaller every second.

When I go back, the frames return to normal.
The object has batching static enabled.

Answer 1

It makes sense that the building has more polygons than empty space.

Answer 2

show the model of the building (polygons should be visible) maybe the issue is in the optimization of the model