Quaternion + many collision meshes = intense lag?

Okay. I've been working on a simple open/close door script that I first found off of YouTube and modified a bit. I will post the code below.

Now one thing that has me utterly confused is why a particular Qaternion.lerp call is bringing my game from 60 fps down to 15 while the function is being called. My scene is very large; 55k trees, all with collision meshes, create the forested environment. If I remove the collision capsules from the trees via the terrain editor, the lag vanishes.

I'm a bit surprised because I would have thought that it's the onTriggerEnter and onTriggerExit that would be the costly bit of my script below--why whould Quaternion.lerp take into consideration collision meshes at all? The doors are not rigid bodies; nothing can (or is supposed to) stop them from opening.

Can anyone provide some insight as to what is happening and why? When I inevitably use AI calls with Quaternion.lerp or Quaternion.slerp, this issue will probably come right back.

 // Smothly open a door
 var smooth = 2.0;
 var DoorOpenAngle = 90.0;
 var DoorCloseAngle = 0.0;
 var open : boolean;
 var enter : boolean;
 var moving:  boolean;
 var squeak : AudioClip;
 
 var rotateTimerCurrent = 0; 
 
 var myTransform;
 var target = Quaternion.Euler (0, DoorOpenAngle, 0);
 var target1 = Quaternion.Euler (0, DoorCloseAngle, 0);
 
 function Awake() {
     myTransform = transform;
 }
 
 function Update (){
 
 // rotate only if door is not fully open or closed, kept track via rotateTimerCurrent
 if (rotateTimerCurrent > 0)    {
 
     if(open == true){
         myTransform.localRotation = Quaternion.Lerp(myTransform.localRotation, target, Time.deltaTime * smooth);
     } else {
         myTransform.localRotation = Quaternion.Lerp(myTransform.localRotation, target1, Time.deltaTime * smooth);
     }
     
     //decrement rotation timer
     rotateTimerCurrent = rotateTimerCurrent - Time.deltaTime*.1;
     
     }
     
     //check if player is trying to open the door
     if(enter == true){
         if(Input.GetKeyDown("f")){
             open = !open;
             audio.PlayOneShot(squeak);
             rotateTimerCurrent = 40f;
             }
         }
 }
 
 //Activate the Main function when player is near the door
 function OnTriggerEnter (other : Collider){
     if (other.gameObject.tag == "Player") {
         (enter) = true;
     }
 }
 
 //Deactivate the Main function when player is go away from door
 function OnTriggerExit (other : Collider){
     if (other.gameObject.tag == "Player") {
         (enter) = false;
     }
 }