I happened to put 50 2D objects (that all have a rigidbody, a collider, and a script telling it to move in a random direction) in the same spot, and when I ran it, I discovered that doing so causes a huge lag until they start to spread out. If they are not in the exact same spot (even by less than a unit), no lag occurs.
Physically two bodies cannot occupy the same space at the same time, so you’re trying to break physics. Joking. Yeah, that’s normal. If you really have the need of having so much 2d rigidbodies at the same point, something you can do to improve your performance/experience is to use layers. Another thing you can do is to disable rigidbody2d and collider2d to those objects, and one by one enable those components and move it randomly in different times.