Level of Detail on meshes
Large models made of many meshes cost frame rate. Level of detail (LOD) buys it back: VRGS builds progressively simpler copies of a mesh and swaps to a simpler one as the camera moves away, so distant geometry costs a fraction of what it does up close.
Right-click the mesh → Optimization → Level of Detail (LOD) → Generate.
The switch is driven by average triangle size on screen: when the current level's triangles shrink to about one pixel, the next level takes over. Choosing the moment that way is what stops the transitions from visibly popping.
To watch it work, switch on Wireframe in the triangulated mesh group properties and navigate towards and away from the model — the triangle density visibly changes at each transition.
Optimization → Level of Detail (LOD) → Remove deletes the layers again.
Editing the geometry afterwards — decimating, remeshing, deleting triangles — invalidates the layers, and you have to generate them again.
More on this, and on tiling for models too large for LOD alone, in Working with Large Models.