The problem is in this CSS resource:
http://dnqgz544uhbo8.cloudfront.net/_/fp/css/main-base.f4gmHr37dkTU585y-PwfDg.css
Specifically this line:
.wf-loading .post-header .post-title,.wf-loading .post-field.subtitle,.wf-loading .post-field.body,.wf-loading .post-meta,.wf-loading .post-author-card,.wf-loading .post-article .caption{visibility:hidden}
And specifically this part of that line: .wf-loading .post-field.body,
I verified this by downloading their only two CSS resources locally and along with an HTML page from their site. I edited the HTML to point to my local CSS from their CDN and could reproduce the slowness every time (loading from my local SSD!). When I remove ".wf-loading .post-field.body," from the above-mentioned CSS, the page loads instantly.I figure I could dig some more to see how that CSS class is being used any why it's being used. But I figure this is a good task for someone at Medium.com ... of course this may be a Mozilla bug. And the real defect may be in something that's slow prior to changing visibility from hidden. Anyhow, if a developer at either organization could fix this, I'd greatly appreciate it. Maybe some others experience this same bug.