Of course, it was incredibly slow in a more important sense: the latency with which you'd see responses to your messages. Your reply had to propagate through the network of Usenet peering relationships before your counterparty could see it. Contrast that to "primitive" web message boards, where that propagation is instantaneous.
But, most importantly: because everyone was keeping a replicated log of everything anyone had ever sent, almost nobody had archives. The only way to read old forum messages was to... wait for it... go to a web application like DejaNews.
Software piracy killed Usenet --- binaries made it impossible for a typical provider (of any sort) to provide full-feed Usenet, which is what customers demanded, and so everyone gradually migrated to centralized providers. But even if that hadn't happened, Reddit would have killed Usenet eventually.
(I ran a Freenix-ranked Usenet server in the 1990s for a mid-sized regional ISP, in part by independently inventing the INN history cache; my love for NNTP is deep and real.)