Thank you for your comments.
I thought the point of building a list of nodes to be tracked was so that one could be told "here are the new replies to any of your tracked nodes" (either with one page load or via notifications sent out by some cron job).
It's possible. This requires 1 call to the XML Node Thread ticker for each thread being tracked, but only a single a single call to the node query XML generator. The sorting out of what node belongs in what thread would be done on the external server. The only way I can think to limit the impact is restrict the number of nodes that can be tracked at any one time.
Using your current technique of grabbing information, adding such a feature would not be an efficient use of the server. It would refetch the entire thread list for each interesting thread (and fetch that separately for each user who wants to track the same thread) every time someone wants to check.
I guess my vision of using this tool is slightly different. Users that didn't login for weeks could still see if there were any updates to nodes long gone from Newest Nodes. Since there is not a constant check for updates, the only time the time PerlMonks gets queried is if the user runs a report. I also envisioned different users tracking nodes at different rates.
Cheers - L~R