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Nielsen’s first heuristic, to my knowledge, indicates that the interface must continually inform the user about the status of the application so that the user always knows what is happening.
That is, it specifies nothing about the details of what is happening.
An icon of loading without a percentage sign "hurts" in a certain way this heuristic when informing the user only the status of the system, without detailing how much of the task has already been executed or how much time remains for its completion?
I voted to close as duplicate, but more precisely your answer is where they speak Loader Undetermined x Determined there is a lot of detail on this question of your question for both UX and UI
– hugocsl