StaticMeshInstance performance concerns

I am drawing 5k static mesh INSTANCES, with NO actual mesh attached (just a test, normally there would be), and yet with no mesh at all, the framerate drops from around 70fps to around 10fps. This is in the editor, not a game. What is it about mesh instances with no actual render component that is causing such a huge perf hit?

EDIT: Oh interesting. I have a variable in the blueprint which is an array of transforms (one for each instance). That was “Editable” but the 3d widget was disabled, so nothing was visible in the viewport. However if I mark it as not editable, the perf jumps right back up to where it should be. so IOW havign that large array as editable is what causes the hit.