I think the problem is that you click on “all” when opening mega threads and what that does is make the whole thread into one single page and that can make your browser seem long.
The problem can't be with the "show all".
OP referred to Wall Observer thread and as I said in my previous post, the "show all" button isn't available for the threads with more than 25 pages at all.
You can see all posts of a thread in a single page, only if it has 25 pages or less.