Open world optimisation

At the expense of “World composition” I created a huge world, placed NPC spawners, but because of their number, the FPS drops. How can I hide objects that are far away from the player? Rather, not just hide it, but improve performance. Meshes have LOD’s, textures are optimized, but this is not enough.
Sorry for my English.

Super easy fix would be to add a depth based fog and then simply anything past the density of the fog is set to be inactive or to not render (hidden in game).