I cannot understand why this is marked as not "Not Reproducible", when it is so easily reproducible, just create a simple scene, create a cube, attach a rigidbody to it and hit play... that's enough to make unity 2018.1.b crawl at a painful 15 fps or less. Disable the box collider and performance improves to about 30 fps.

I am getting this issue. Even with no colliders in the scene.
They really need to get a fix for this because I can't develop my game with it there.
I believe the issue likely arrives from using multiple threads. Even when the Thread is not doing anything.

Why so few voites? Very critical bug!

This Issue ID760561 is the same problem of my Case #717222 (from2015) reported closed by Roman "this case as Morten already took the initiative on the forum post." But i have made the same report since Unity 4.1 and it looks like persist in all Unity versions, In Windows 7 and 10. And persist in a bast range of platforms (in my case since i3 to i7).

I reported this SPIKES since Unity 4.1 later 5.1 and persist in Unity 5.4.beta17
I think this bug is deep into Unity code and an old one. Good chance is not in the physics process but in rendering or else and physics Inherited the problem. Can be track to the origen in code of Unity 2 or Unity3 in an empty scene?

I get this problem in Unity 5.2.3p3 it's a show stopping bug that stutters and lags gameplay. I've found the spikes are present still in a new project in an empty scene, my post on this here http://answers.unity3d.com/questions/1157658/huge-physics-spikes-in-empty-scene-and-populated-s.html

Simple way to reproduce it. Create Thread with empty "while" loop.
private void Start()
{
Thread tr = new Thread(Job);
tr.Start();
}

private void Job()
{
while (true)
{

}
}