Nanite and UE5 performance issues (low poly, many instances)

Basically i have a large blockout scene in the viewport (made with Blockout Tools Plugin), - about 6000 boxes (Nanite enabled) plus about 500 translucent boxes (tried to make them opaque, almost similar performance).

And here is how Nanite handles it:

So i’m getting around 25 fps (rtx 2060).
Without Nanite, performance is a little better (40 fps).
View mode is UNLIT (so no lighting).

Also there is a simple landscape on the scene, volumetric fog and sky atmosphere.
Virtual Shadow Maps OFF, Lumen OFF, RTX OFF.

Why so bad performance?

Also it takes almost 5 gb of VRAM. There are literally 6-7 textures on the scene. Is it some kind of memory leak?