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I've come across with the sentence below?

Old tasks become easier the second time around, but it doesn’t get
easier overall because now you’re pouring your energy into the next
challenge.

I don't know what the reference of "it" is here. The tasks? the process? or anything else?
So could you please tell me what the reference of "it" is here?

The full text is:

Usually, this minor dip in performance is no cause for worry. [...]
The less energy you spend on trivial choices, the more you can spend
it on what really matters. However, when you want to maximize your
potential and achieve elite levels of performance, you need a more
nuanced approach. You can’t repeat the same things blindly and expect
to become exceptional. Habits are necessary, but not sufficient for
mastery. What you need is a combination of automatic habits and
deliberate practice. Habits + Deliberate Practice = Mastery To become
great, certain skills do need to become automatic. Basketball players
need to be able to dribble without thinking before they can move on to
mastering layups with their nondominant hand. [...] But after one
habit has been mastered, you have to return to the effortful part of
the work and begin building the next habit. Mastery is the process of
narrowing your focus to a tiny element of success, repeating it until
you have internalized the skill, and then using this new habit as the
foundation to advance to the next frontier of your development. Old
tasks become easier the second time around, but it doesn’t get easier
overall because now you’re pouring your energy into the next
challenge. Each habit unlocks the next level of performance. It’s an
endless cycle.

Atomic habits by James Clear

That is rather like a "weather it". The sentence requires a subject and "it" is used without a definite reference.

You could interpret "it" to mean "The process of developing mastery", ie the subject discussed in this paragraph.

Let's say a teen is complaining to a parent about how difficult high-school is.

The bus comes so early, at 6:15AM. I've got late lunch, so I'm starving by then. And Mr Jones is so boring.

The parent might say "I've got news for you: it doesn't get any easier". There "it" alludes vaguely to "what is expected of you" or "life in general" or "things".

It does not always refer to a specific noun. It can refer to the topic of conversation, the context. Using it in this way is conversational.

This paragraph refers to becoming great at some type of activity (they give the example of Basketball, but it can be anything). To excel in some activity, you have to learn how to excel in many small tasks. But each time you master some aspect of that activity, becoming great at that activity doesn't get easier, since there is always the next aspects of the activity to put your efforts in mastering.

The "it" in "it doesn’t get easier overall" refers to becoming great in the field.

That sentence demonstrates the use of an impersonal verb in English. English is strict about always requiring a syntactic subject, even if semantically none is required; the pronoun it fills that need.

Your "full text" starts with "Usually, this minor dip in performance is no cause for worry. " What is this "minor dip", and what in what is the performance? Almost certainly, there is more to the text, so the answer to your question depends on that text. Let's say the text says:

When you're learning a new skill, you continuously master more tasks. Old tasks become easier the second time around, but it doesn’t get easier overall because now you’re pouring your energy into the next challenge.

Then "it" refers to "new skill".