There's not much I don't like about the way the LotusScript (and Java, and COM) "back-end classes" for Notes and Domino work, but there are a few. Chief among that very small list is the call NotesDocumentCollection.GetNth(). It's a performance killer (most of the time, for large collections).
I started talking about that, writing about it and generally warning people about it over 10 years ago. By now, for me, it's kind of a boring topic, but I still get questions about it, and I occasionally see misleading or mistaken things written about this topic. So, here are 2 links to posts on this topic on my blog. The first,
is an in-depth discussion of what's wrong with the implementation of that call, and why you shouldn't use it, including a sample LotusScript program that demonstrates the problem. The second link,
is a description of the ONE time I really needed the GetNth call, and used it even though it's a lot slower than the alternatives.