DX11 renderer slow-down from large geometry?

Just noticed something that doesn’t make sense to me in an obvious way.

I have a DX11 renderer to render lots of 3D objects. The objects are just simple boxes of different sizes. I am noticing this when there is about 2000 of them, most are intersecting in some way. I am rendering with MREmdmod.

Everything is running smoothly at around 30fps when the boxes are small, but when i increase their sizes the framerate starts dropping sort of linearly down to 2fps once they are quite large (start getting near and far clipping).

Maybe I know too little about 3D rendering, but I wouldn’t expect the framerate to drop so dramatically.

Anyone got an idea why its happening and how to avoid it? Thanks.