Well, a forum is essentially the same thing as BBSs were in the 80s but now we have computers that are several thousand times faster. I find it much more "amazing" that the other forums are so slow.
BBSs in the 80s / 90s were operated by users in their own homes and usually weren't allowing for multiple connections. The software itself was pretty dumb and slow.
Forums these days need to allow for multiple users to log in, read content and post messages. I'm pretty sure that the link above received several hundreds or thousands of users in a single hour. Forums these days also use database systems (or other services external to the process) that represent a big bottleneck. Then you've got the issue of static assets, like CSS, Javascripts, images and so on.
As a general rule of thumb, if you want scalability, you have to trade some throughput for it, as scalability concerns hurts performance.
That said, popular open-source web software, in general is awful in terms of everything.
