App/game freezes completely while coroutine runs - unity3d

Question

My Android ARCore Unity app/game freezes when a coroutine runs, and then resumes once the coroutine finishes. The coroutine is in a script attached to a gameobject, and the coroutine is called in the Start() function. I have tried inserting yield return null; into more parts of the coroutine, which reduces the time of the pauses, but adds more pauses, resulting in very choppy (basically nonexistent) gameplay for a second or two after the object is instantiated. Could this be happening because ARCore does not support multithreaded rendering? If that is the case (or isn't) how can I fix this, so my coroutine runs (I guess 'in the background') after the object is instantiated and so it does not affect gameplay FPS?
Here is my code:
void Start()
{
StartCoroutine(MyCoroutine());
}
IEnumerator MyCoroutine ()
{
yield return null;
Transform[] allChildren = GetComponentsInChildren<Transform>();
foreach (Transform child in allChildren)
{
if (child.gameObject.GetComponent<MeshFilter>())
{
if (!child.gameObject.GetComponent<MeshCollider>())
{
child.gameObject.AddComponent<MeshCollider>();
}
}
child.gameObject.tag = "CenterObject";
}
}

Answer 1

Coroutines seem to have an overhead when constructing them as discussed here: https://forum.unity.com/threads/startcoroutine-performance-cost.233623/
Given that you're instantiating objects I would suggest these paths:
Change the coroutine into a simple function call, since you don't seem to be requiring any delay between iterations of the loop which is the primary idea of using a coroutine in Unity. You are also adding colliders and expecting gameplay to continue, which can cause unexpected behaviour if you need collisions to be registered.
Instantiate prefabs which have colliders attached beforehand, if possible.
Use object pooling where you would create the objects before gameplay starts and activate them when required, and disable them when they are destroyed, if necessary.

Answer 2

It's because you are caught looping within the coroutine. You need to move the yield to inside the loop:
IEnumerator MyCoroutine ()
{
Transform[] allChildren = GetComponentsInChildren<Transform>();
foreach (Transform child in allChildren)
{
if (child.gameObject.GetComponent<MeshFilter>())
{
if (!child.gameObject.GetComponent<MeshCollider>())
{
child.gameObject.AddComponent<MeshCollider>();
}
}
child.gameObject.tag = "CenterObject";
yield return null;
}
}
Even so, if this is a large number of children you are looping through, it may simply be a performance issue.

Answer 3

In addition to the answers fixing your Coroutine:
Coroutines are not really multithreading! That's why your app freezes if there is a long task in the Coroutine.
The yield return of Coroutines just makes it easy/lazy to do a quick animation or something similar because the Coroutines kind of "remember" where they left and go on from this point (to say it in simple words) so you can run a lot of things apparently "parallel". But still it all runs in the same thread so actually it only seems to be executed parallel because everything is executed between two frames.
If you realy have a time consuming task you want to do in another thread than have a look into async await which is Unity's real multithreading since Unity 2017.
However Unity is not thread save meaning most of the API can only be used in the main thread...