I can also visit Wall Observer thread and scroll its pages without any issue.
I don't think it matters how large a thread is. In every page, you will see the maximum number of 20 posts.
Did you mean about Show all in huge (mega) threads?
I don't think OP is referring to "Show all".
"Show all" button is available only for pages with less than 25 pages and not for a very large thread like Wall Observer.