I can manage all my collections in my account on amazon. Its been a feature for some time. At least in the US. I don't know about other countries. I have like 45 collections with many books in some of them. I am talking 200-600 books or so. Doesn't take more than a second for me to add or remove. Maybe 2 seconds on the largest collections.
I been buying kindle books since 2008 so I have a few now.
