collider2D rotating in 3D = very slow

If you want a Collider2D to move you should be moving the body and not the Collider2D itself. If you move the Collider2D then it has to be recreated. The same goes for performing Transformation; do not perform 3D rotations on 2D colliders. PhysX has a transform for this, Box2D does not where effectively any collider shape(s) are immutable and directly live in the space of the Rigidbody2D which is the only thing that should be moving. The same goes for simple things like (say) changing a radius of a circle; that means the circle has to be recreated.

2D is already pretty optimized in terms of avoiding recreations so it'll only perform them if it's absolutely necessary; rotating a GO in Z with a Rigidbody on it will not cause any of its connected colliders to be recreated. Rotating in XY will (not that circles or capsules will rotate in 3D anyway). All scaling operations will cause a recreate. The same goes for any properties that change the collider geometry. Anything else causing recreation is a bug!

Also, it won't take 12ms to "rotate a few circle 2D colliders"! In the profiler you're showing this being done thousands of times. Destroying shapes has a larger overhead then creating shapes because there's extra work being done to ensure that when the shapes are recreated, any new contacts on the recreated shapes are paired with the old contacts so you get enter/stay/exit callback consistency. This is a PITA to do but without it, you'd just keep getting enter callbacks.

FYI: There's work in the planning stages to rewrite large parts of Box2D to get around a bunch of things that are a pain. One of them is this issue with immutable shapes and the other is contact reporting and handling. This work won't start until much later in the year however.

If you have a simple reproduction case, I'd be happy to take a look at it.

 

I checked out your project. So not a "few Circle 2D" then, actually hundreds each frame and also you're only rotating in Z and not all three axis. It's really not clear then what you want.

So back to the project; you are not rotating the Rigidbody2D at all, you are rotating its child hierarchy so colliders are being moved releative to the body, which begs the question why you've even got a Rigidbody2D at all. In your project, you are rotating a GO named "pivot" in Z only. If this is the only GO rotating in Z then add the Rigidbody2D here. This will rotate the Rigidbody2D only and not cause the Collider2D to be modified because they live in the space of the Rigidbody2D and in 2D, a rotation in Z (as well as translation in XY) are fine.

In summary, move the RB to the "pivot" GO.

Note however, if you're going to rotate the GO in XY (which is a bad thing to do and doesn't even make sense for some colliders like circles) then you'll get recreations. To be absolutely clear: circles do not rotate in XY as these as 2D colliders, not 3D ones. The vertex based ones don't rotate either, they just have their verts reprojected into 2D after rotation. This is expensive and you need to be sensible and use XYZ translation and Z rotation only for 2D and avoid scaling.

 

Sure I understand but I'm re-enforcing why you should not attempt to rotate the colliders. If you have a 3D asset on a GO hierarchy that must rotate, do it on a different part of the hierarchy and ensure you don't rotate the 2D colliders that way.