I use NB on a PC with a 3570k and 8GB of memory, running W8 64, minus the Metro.
In the short time I have used NB I found that beta 2 seemed to load all the headers fine when, for example, I downloaded 5,000,000 from each of the groups that I had combined within one folder, if that makes sense. .? (So if I had a folder named "Cars" then two newsgroups related to that would be within that folder).
So to load all headers from the disk and display them all worked a treat, and was pretty responsive.
With beta 3 it seems to be really really slow to the point of hanging the application when trying to do the same thing, with a large number of posts from several groups. Several times I have purged the groups of all headers, thinking that they have got corrupted, but the same applies.
If I download a period of 15 days or so it will often work fine.
I think that another user here is experiencing the same thing...
viewtopic.php?f=43&t=32111EDIT:
I have just purged two groups and downloaded 5,000,000 header in each (from a "master " folder) then I right clicked on that folder to display all the posts and it took around 20 seconds to display all of them going back to the 16th of March (78,883 posts). So maybe it depends on the number of posts and groups you are trying to display in one go before it becomes an issue..?