I'm struggling to understand the difference between говорить/сказать and говорить/поговорить. 

I've been making notes from the Master Russian website, which lists говорить/сказать as meaning to say, tell, and говорить/поговорить as to speak, talk, say, tell, or, talk-for-a-while. I understand that говорить is the imperfective aspect and that сказать and поговорить are the perfective and that говорить, сказать and поговорить have their past tenses in говориbл, etc., сказал, etc., and поговори́л, etc. So my questions are:

1) What's the difference between сказать and поговорить as the perfective aspects of говорить - especially in the past tense?

and 2) How can поговорить as the perfective aspect of говорить suggest to talk-for-a-while when to talk-for-a-while seems far more process-orientated and, hence, imperfective than perfective? 

It's important to note that I'm aware that there exist a question on the difference between `сказать` and `поговорить`. However, the answer that was given focused on `поговорить` in the infinitive and not the past tense. I'd really like to understand how they differ from one another when conjugated.