Generally, I write less on drafts than I think many of you do because I focus on one or two key points to revise. That means that my marginal comments on the portfolio final drafts are likely to be fuller than the ones you write. If you've already given extensive feedback in the margins on the drafts, I don't think you need to repeat all of that on the final drafts.
I write one end comment for the portfolio, and I typically use a "sandwich" approach. I start with the overall strengths of the portfolio, particularly those strengths that appear consistently in all (or most) of the papers. Then I typically include a short review of the key points for each paper. Next I focus on skills students should concentrate on for the next portfolio (i.e., weaknesses to revise for next time). Finally, I try to list the specific skills--usually demonstrated strengths again--they can build on for the next portfolio.
I always write my end comments on the computer, and occasionally I can cut and paste sections from one end comment to another, particularly that last chunk of strengths/skills to build on. That can save a bit of time.