Low Fps After Unloading Tiled Landscape

I have a 12km x 12km tiled landscape. When all tiles are loaded in the editor, my fps drops, which is to be expected. However, when I unload the tiles in the levels window, my fps remains the same as if they are all still there and being rendered. It will return back to normal once I restart the editor. It also made creating landscape LODs difficult because my fps would drop to 6-7 as if it was rendering the entire landscape with full detail plus the LOD tiles. Once again, it would return to normal once the editor is restarted. Is this normal behavior? I have a GTX 1080ti and an i7 8700k cpu, so I shouldn’t be too limited hardware wise.