I disagree. I find Discourse superb. From the user’s perspective it is simple, clean, fast, with amazing search, etc. It can be customised, but most people using it do it for practical reasons, and worry little about customisations.
I experience it as fast compared to traditional, pre-SPA-era web forums; the use of JavaScript speeds up a lot of interactions that would otherwise require a full page load. The live Markdown preview is also a heck of a lot better than having to click “preview” and wait for a page load to see your BBCode rendered. Having live updates for new posts/edits without having to refresh contributes to a snappy feeling. And I think the visual theming is nice and clean.
Still, compared to a good email client, it’s not so fast. Which is why I personally read Discourse forums mainly via email, only visiting the actual site for long threads or when I want to reply. It’s not perfect, since Discourse allows editing posts but doesn’t notify email users of edits. But it’s fine. I actually like it better than real mailing lists, because I prefer Discourse’s composing interface.
Discourse is definitely not fast. Whenever Google sends me to a discourse page I just end up giving up on waiting for it after staring for over 8 seconds at a blank page.