Why is my meshes taking to long to merge?

Hello, quick question, I have always been wondering why it takes so long to merge my meshes together.
For example, I’m using 8k textures, and nanite meshes to create a box as shown in the picture.

Is there something wrong with what I’m doing, and is there anything I can do to fix this?
Because after merging, my fps is significantly dropping from 120 to 40