BPs to HISM Performance Drop issue

Hi! I’ve got a rendering performance issue with blueprints / HISM (Hierarchical Instanced Static Meshes.)
We have a scene with 400 blueprints and everyone of them contains a static mesh. The scene goes 100 fps but if we subdivide the 100 bps in HISM we get 44 HISM and 55 fps (45 fps drop!!)

We tried movable and static meshes, hzb occlusion disabled and enabled but nothing changed.
Is it possible? Why?

Thank you very much!