I am designing a forum with large discussions, with no specific need to order answers. Do I need to implement pagination or should I maintain one unique page per thread?
In a forum thread, users are likely to want to go to the start, or end of the thread fairly often, as well as possibly a particular point in the thread. That makes forum threads a poor candidate for infinite scrolling, as to get to the beginning (or end depending on how you set up your forum), someone will have to scroll through the entire thread. And some threads can get very long.
By way of example, here is a picture of the pagination on a long forum thread:
So for forums with time based threads, pagination is the best current solution.
However for forums where the threads are based on some other criteria to order the most important posts first, this doesn't apply. Take this site or Reddit as examples. Here the posts are sorted (in theory at least) by quality, and so there isn't a problem in scrolling down to get to more content. Reddit just shows a 'load more' option when you get further down.
If you sort posts by time posted, use pagination.
I suggest to split this into two points:
a) Page loading speed and data traffic b) Information context
Page Loading Speed
From a loading speed of view, it doesn't make sense to load all data. Is the user only interested in the latest response (or the few latest added messages), he has no advantage of having all the data on the site.
If the answers to the threads are sorted by date, then the user should be able to jump directly to the latest answers (or sort according to date from latest to earliest). If the answers are not sorted in any specific order (normally they are anyway based on the date they got created), then I would suggest to automatically load the answers (lazy loading) as the user scrolls down. Asking the user to interact explicitly with such a paginator control is from my point of view really annoying.