In my game I have about 10 Actors that happily move around the map and to their things. They have skeletal meshes and animations. This all works fine with ~60 FPS. Now when I scale all those 10 actors to 3.0 (instead of default 1.0), then the performance is quite bad, I have only 35-40 FPS.
How could the scaling of Actors have such a huge impact on the performance?
If uniform scaling, it shouldn’t hit performance; non uniform actor scale may hit rendering performance a bit yes; but you shouldn’t see a drop in framerate that big, may be there other issues affecting your blueprint as well.
Well, I changed only that single thing to test 
When i use big upscaled meshes, i get sometimes a UE4 warning about performance drop, when dynamic shadows are active, or similar.