This idiom has a positive connotation indicating confidence, experience or proficiency of the subject. So, the second one it is. I agree that a talent for maths is a good context in which 不假思索 is used.

"不假思索地/不假思索就“ is an adv used to describe someone who is decisive or confident that he or she would make a right move without thinking about it, which may be because of his/her past experience or intelligence or other unknown reasons. eg. 他的心算能力很好,一看题便能不假思索地/就说出正确的答案

”不假思索” we usually use it to describe people saying something or doing something very quick. It emphasises the action . Mostly people "不假思索” do or say something that because they have had the anwswer in their minds.They do it automatically or without hesitation. There is another situation is that people do or say someting quickly without thinking,this may cause them regret.It all depends on the context.

The second one.

It means that one can do something quickly without thinking because they've done it so many time before/ they are very familiar with the concept/ they've thought this through/ they think it's kind of normal sense.