Awake() and Start()

Awake is the equivalent of the ctor. It is called when a MonoBehaviour is created, before any other method if the object is active.

Start is run the first time the MonoBehaviour is activated. This can be right after Awake or long after. This allows to perform actions that are related to the current state of the app or objects.

For instance, you create an enemy, in Awake, you place everything that is basic initialisation. Then, the enemy is deactivated at the end of Awake. Later on, the enemy is about to be activated but you want to make it red if player is having some specific weapon, then you do it in Start.

IMPORTANT: If a prefab is created and the game object is off by default in the prefab, the Awake is not called until set on. In the case of a pool creation where prefab may be off, the awake is happening first time the pooled object is activated.

OnEnable is similar to Start but happens on every SetActive(true) and on start if enabled. This can be a candidate for your enemy willing to change color over the level based on the player magna for instance.

Usually Awake() is used to initialize if certain values or script are dependent on each other and would cause errors if one of them is initialized too late (awake runs before the game starts). Awake is also called only once for every script instance.

Let me quote the Documentation:

[...] Awake is called after all objects are initialized so you can safely speak to other objects or query them using eg. GameObject.FindWithTag. Each GameObject's Awake is called in a random order between objects. Because of this, you should use Awake to set up references between scripts, and use Start() to pass any information back and forth. Awake is always called before any Start functions. This allows you to order initialization of scripts. Awake can not act as a coroutine.

and about Start():

Start is called on the frame when a script is enabled just before any of the Update methods is called the first time.

Like the Awake function, Start is called exactly once in the lifetime of the script. However, Awake is called when the script object is initialised, regardless of whether or not the script is enabled. Start may not be called on the same frame as Awake if the script is not enabled at initialisation time.

Where the last part makes one big difference

To get to your question:

If the script is NOT enabled at the beginning of your game, and you don't need the variables to be initialized, start would be saving performance as awake() would be called regardless...
every variable would be initialized at the very beginning. At least that's the logical assumption I make.

This topic is well described in the official docmentation (Awake and Start).

This section describes why you might need two functions:

The Awake function is called on all objects in the scene before any object's Start function is called. This fact is useful in cases where object A's initialisation code needs to rely on object B's already being initialised; B's initialisation should be done in Awake while A's should be done in Start.

The difference between Awake and Start is that Start is called only when a script is enabled.

These two functions are called before the first Update method and there is no performance difference between them. I would say that Awake is used to initialize all objects (like a constructor), and Start is used to link the objects or do something before a game starts.