Win10 is the worst to work/play with all programms which are older than Win10 and the worst for 70% from all which are newer. So don't blame the Editor.
Making such a big and detailed map which you has opened there need a few more hours of work.. 1000 and more.. so as long you don't made something like that from scratch yourselve, you don't have the right to say that they are "poorly made".. also when they are. It can have a bad gameplay, but thats an other point. You will have bad gameplay when you play with 16 vs 16 players on a map which is made for 3 vs 3 and vis versa..
The Editor is still a 3D-Modelling programm and they are all bad, the old as well as the new. The bigger the map, the more details in your view the slower it is.. using win10 makes it just worse. The Engine itselve could also handel high poly StaticMeshes, and with the right lightning it can nearly look as good as the newer engines (And it will also heat up your CPU as hell.).
Try using an other view (Textured), remove the SM (W) the Volumes (O) the Fog (F) and "Hide Actors". That should make it faster.