If you use "question by question" mode you can see how long it takes for the respondent to move from one question to the next question (which is slightly different o "how long it takes to answer the question", but should be pretty close). If you use group by group mode, this is not possible out of the box, because limesurvey only saves the timings when a page is submitted and not when something is clicked. Actually, this would be pretty dangerous, because what if someone clicks one answer (time saved) and then changes the answer? You would have to cater for this as well and things would get quite complicated.
In group-by-group mode I guess you could create hidden questions where you write timings via Javascript, but this would be quite some work if done for all questions, I guess. And what if someone answers the second or third question before the first? Or first reads all questions, then starts to answer, or....
I don't see a good way to measure question timings in a group by group setting reliably. And if it is not even close to reliable, I don't think it makes sense to use it.
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Note: I answer at this forum in my spare time, I'm not a LimeSurvey GmbH employee.