Unity C# - Radar works but performance is horrible (Update & LateUpdate)
Let's review the code from bottom to top.
Remove_PositionIndicatorOnEnemies

Having an underscore in a method name doesn't fit with the .NET naming guidelines.
By reverting the first if condition you can return early if enemyListHolder.activeInHierarchy is false which saves you one level of indentation.
The next if condition can be omitted because that check is done inside the loop condition.
Instead of using a for loop I would prefer a foreach loop and would restrict the items to only them where activeInHierarchy is true by a Where() clause.

Implementing the mentioned changes will lead to
private void RemovePositionIndicatorOnEnemies()
{
    if (!enemyListHolder.activeInHierarchy) { return; }

    foreach (var enemy in enemyIndicList.Where(e=&gt; e.gameObject.activeInHierarchy))
    {
        enemy.gameObject.SetActive(false);
    }
}  

Add_PositionIndicatorOnEnemies
Here, the first three points from above apply as well.

The inner for loop can be replaced by FirstOrFDefault() which is more readable.

Implementing the mentioned changes will lead to
private void Add_PositionIndicatorOnEnemies()
{
    if (!enemyListHolder.activeInHierarchy) {return; }

    for (int i = 0; i &lt; EnemiesManager.Count; i++)
    {
        var enemy = enemyIndicList.FirstOrDefault(e=&gt; !e.gameObject.activeInHierarchy);
        if (enemy != null)
        {
            enemy.gameObject.SetActive(true);
            Vector3 aLocatorPos = theCam.WorldToScreenPoint(satManager.GetByIndex(i).transform.position);
            aLocatorPos.z = 0;
            enemy.transform.position = aLocatorPos;
        }
    }
}  

LateUpdate

Again returning early will save one level of indentation.

Implementing the mentioned changes will lead to
private void LateUpdate()
{
    if (!showIndicators_Enemies) { return; }

    if (!enemyListHolder.activeInHierarchy)
    {
        enemyListHolder.SetActive(true);
    }

    Add_PositionIndicatorOnEnemies();
}  

Update

Here we can save two levels of indentation by reverting and combining the first two if conditions.

Implementing the mentioned changes will lead to
private void Update()
{
    if (!statsView_Panel.activeInHierarchy || !showIndicators_Enemies) { return; }

    if (!enemyListHolder.activeInHierarchy)
    {
        enemyListHolder.SetActive(true);
    }

    Remove_PositionIndicatorOnEnemies();
}