I can implement an easy fix which is not very specific about the last edited date: e.g all pages of one topic will use the same date of the last edited post.
Thereby on a new post in a topic (with many posts and many pages) all the pages will get a new last changed date - even when only the last page needs the new date.
Is this a feasable solution?
Otherwise we need to bundle all posts of a topic in packages of 20 posts (per page). And then calculate the last changed date for each package itself.