I don't actually use good reads, but I clicked around - everything I clicked on loaded faster (less than 2 seconds, usually less than 1) than everything I clicked on in medium (always more than 2 seconds, where this blog post is hosted).
Neither site is fast, but "one of the slowest around" doesn't track either.
I think it may be more so for logged in users. When logged in, and I go to the homepage, which is essentially an activity feed, it has 93 requests, 4.4MB of resources (1.5MB transferred), and took 7.66 seconds to finish.
In my experience, Goodreads loads slower on a phone than on a desktop browser (and it's only the initial load that I notice being slow). I've long suspected (without evidence) that this is intentional because they nerf the mobile web experience in an attempt to get you to use their app.
Neither site is fast, but "one of the slowest around" doesn't track either.