Why do you need to use OnCollisionStay? Saving a copy of contacts during OnCollisionEnter could be enough, depending on what you need to do:

var myContacts: ContactPoint[];

function OnCollisionEnter(coll: Collision){
  myContacts = coll.contacts;
}

You should edit your question an post your script - we could check it and have more ideas on how to reduce the CPU load.

EDITED:

OnCollisionStay is called each physics cycle while the rigidbody is awake - even if you disable the script - but maybe you could reduce the load by calling the barycenter calculation routine only at some interval, like this:

public float baryInterval = 1; // report barycenter each second
float baryTime = 0;

Vector3 GetContactsBarycenter(ContactPoint[] points, GameObject target)
{
    Vector3 bar = Vector3.zero;
    foreach (ContactPoint pt in points)
        bar += pt.point;
    return target.transform.InverseTransformPoint(bar / (float)points.Length);
}

void OnCollisionStay(Collision collision)
{
    if (Time.time > baryTime){ // report only when the time has come
        baryTime = Time.time + baryInterval; // calculate new baryTime
        Debug.DrawRay(collision.gameObject.transform.TransformPoint(
            GetContactsBarycenter(collision.contacts, collision.gameObject)),
            collision.gameObject.transform.TransformDirection(Vector3.up),
            Color.green);
    }
}

Physx will still have to calculate the contact points each time, but at least your code will only execute at the defined interval.