The pause before answering does not necessarily mean that the person is telling a lie. In rare occasions, there might be an explanation.
For example, a diabetic with low blood sugar may not be physically able to answer quickly. The range is narrow, though. Even doctors have failed to recognize the symptom (I was present at the time).
Your Partner has Diabetes, YOUR Life just Changed
As important to know as CPR. You are not diabetic but know someone who is — your life may also change
There may be other circumstances for the same thing.