Why does Transform.INTERNAL_set_localRotation() have such an enormous performance impact?

I'm working on a car game, custom made wheel physics. Each wheel has a bunch of gameobjects inside of it, and each wheel raycasts down to handle suspension. Grip and traction is determined by the orientation of the wheel.

When I steer, the wheel turns, and this turning then causes the wheel to give different forces to the car, making it turn.

I turn wheels with: wheel.localRotation = Quaternion.Euler( someVector3 );

Now this is fine when you have a car, or maybe two. Each car calls it about 10 times or so. (Not sure how, each car only has two wheels that steer) Anyway, with a lot of multiple cars, something weird happens.

The calculations really begin adding up. To the point where Transform.INTERNAL_set_localRotation() is called 132 times, and costs 14.36 ms.

Another function, Quaternion.Euler(), also being called 132 times, only costs 0.06 ms.

Why does Transform.INTERNAL_set_localRotation cost two-hundred and forty times as much as any other function?

And more importantly, why does one method, called a bunch of times, take almost as long as an entire frame by itself?

Anyone else got problems with this?
And biggest of all, how do I optimize it?

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Edit: Additional notes:

My wheels look like this:

-Wheel (suspension script + grip script (raycasters))
--SubWheel (wheel animation script)
---HandbrakeBall (box collider that enabled when the car stops)
---WheelModel (mesh renderer)
---Skidspawn (skidmark spawner)
----Particle effect
----Particle effect
----Particle effect
--StopSuspension (sphere collider, keeps the car from falling through it's suspension)
--Sounds (audio source)

It's quite a hefty wheel, and the wheel model even used to have multiple sub mesh renderers, which lagged the wheel animation script with the exact same ransform.INTERNAL_set_localRotation() thing, in which case I was again local rotating a gameObject with child game objects.

Thing is, removing child game objects from the wheel model was easy. They were just meshes. I could add them together in one big mesh. I, however, can absolutely not put the *entire* wheel inside one game object, that I would then rotate.

 

Well, 1 year after I'm also experiencing this issue in my game.

My current test scene is a setup with 32 "tank" objects, When tanks idle I'm at 5 ms per frame, when tanks start turning their turrets (via transform.localEulerAngles) update time increases to 10 ms! Profiler shows that 10% of all time is spent in Transform.INTERNAL_set_localRotation() with 32 calls.

I must be missing something...

 

Do you use PhysX? I heard its costly to manually transform when using rigid bodies.