[Physicist]

But, I've never seen a justification (calculation/prediction/article) explaining either "why" the first stars are different than today's (billions times more massive ? Why?)

Because the universe was far denser than it is today.

or reacted faster than today's stars: going through in tens of years what now takes thousands or millions of years.

That follows directly from the larger stellar mass.

Suggested Google search term: "Population III"

[Robert A Cook PE]

Thank you for the reference.