I’d say it’s idiomatic. Metaphors involve a kind of analogical thinking that doesn’t seem to be present in this expression.
I assume “taking her time” is somehow derived from take having the sense of require, as in “it takes her half an hour to get ready”, which I would say is neither metaphoric nor idiomatic.
As in…“She takes forever to get stuff done”?
It can be used as a negative, “He just takes his sweet time no matter who’s waiting on him.”
Or if a person is struggling one could say, “It’s OK. Take all the time you need,” to reassure them.
No it should not imply a requirement for more time, or a need in general for time per se… But the choice of having the amount of time you would like [to get it done well] or [get it done with less stress etc.]
I think that these different interpretations are acceptable uses though! I had not thought of the other potential uses…