Too bad a real solution hasn't been found yet. I am having the exact same issue.
iPhone (iOS 11.2.6). @markus i tried spiegel.de but it takes ages to load on my iPhone. I am not German, but found that heise.de is about the same story. I often use nu.nl (Dutch news website) and it works, but also keeps on loading something for ages.
I have the iptables rules in place. On my laptop (also configured to use Pi-hole) the website spiegel.de is working fine. I really think it is something in iOS that is messing things up.... but what?
I also posted my question on Reddit: https://www.reddit.com/r/pihole/comments/87liyo/some_sites_hangkeep_loading_on_iphone_https_issue/