In my experience, a lot of comments on blogs load slower on blogs which use a third party commenting system like Disqus commenting compared to the commenting system which comes built-into blog software like WordPress which seems to load faster. Reddit comments also load quicker than some blog comments which can take many seconds or minutes to load.
Disqus comments seem to have a loading circle more often these days when I visit a site which use Disqus.
I think the main reason for comments loading slower is because a lot of comments are hosted on third-party servers instead of on the same server like most forums where replies are hosted on the same database server as the topics.
A lot of third-party commenting services maybe overloaded because they host many comments for many blog sites.
I notice Blogspot, Wordpress, and Tumblr default comments load as quickly as the site's load time.