Does it also happen when you switch to Unlit Mode? Try grouping some unneeded items into a folder in your Scene Outliner and visibility icon to hide the entire group.
Also, for first performance check, change to Lightmap Density, notice my closest mesh that’s red color?:
Convert **brush **to static mesh, and then convert the static mesh to a Blueprint, for some reason blueprints seem optimized for performance, and can often solve lighting issues.
Sometimes it works, sometimes it doesn’t and the color will stay red.