This is the first HN comment I have seen that actually describes what about the new design makes it unusable. Most that I have seen are "The new design is unusable! Everyone agrees with me! I use old.reddit.com and do not use the new design!" A few comments point out that nothing loads without errors, but that's not a design problem -- that's an implementation problem.
With that said, I am throwing my hat into the "I disagree" camp. I do not find the new design unusable. It is not slow for me (American cable ISP, Firefox browser, uBlock Origin enabled, JavaScript enabled). My experience is just as fast as the old design. I have direct access to both the shared link and comments (the title now goes to the comments and the shared link is the orange link to the right of the title). I admit I did have to retrain my brain to the new design's conversation-focused design vs. remote-link-focused design, but that took all of a week. I do not have any slowness managing text formatting, but then, I do not comment a lot and I do not write long comments. I appreciate that the new design, on most subreddits, appears cleaner and the information, for me, is more clearly laid out. To me, the site appears "prettier." With the ad-blocker on, some of the sidebar ads get blocked. I do not know how that impacts my experience.
I have also not experienced any of the page loading errors that people have described. Not once, unless it was a known outage causing the Internet to collectively scream.
I am mostly a reader on reddit. I visit my subreddits twice a day, during what are probably peak usage times in North America.