Probably because the light node is more performance intensive, so many of them are lagging it up. Its probably normal, I think most dynamic lighting engines (2D or 3D) have limits on lighting due to the processor complexity.
What you might be able to do to get something similar without using lights is using an additional Sprite node instead of a light. Then in this sprite you can assign it a glow texture and set the CanvasMaterial mode to add
, which should give a similar effect to the light node without as much of a performance hit.