My entities are composed by several sprites which are the parts of the animation. So I do not use flipbooks. I use the sprite collision polygon, adding some vertices -but not a lot-.
I will tray switching to boxes for testing if the penalty is due to geometry complexity!
Regarding to constraining physics, I think i have constrained physics in Y plane in project settings and I think I have checked “Can use 2DPhysics - Box2D” to true in project settings also. BUT I think each sprite has checked the “use 3d physics”
Also, each sprite of my entities is tweened -driven by a UTimelineComponent-, so every frame every sprite can potentially -and likely will- update its transform