In my simple way of putting it - creating a website, or creating a post, and then having it disseminated is dirt cheap today. You can get an article on a news website, having it referenced by a youtube channel, have that sent to a twitter feed.
That alone is sufficient to discuss an increase in the volume of content being created - however, that volume also gets disseminated as fast as it is created, which is what accounts for the speed.
This is also without looking into the fact that people tend to use superficial traits to assess whether information is credible online.
"Yet, research shows that people rarely engage in effortful information evaluation tasks, opting instead to base decisions on factors like web site design and navigability. Fogg et al. (2003), ... They argue that because web users do not often spend a long time at any given site, they likely develop quick strategies for assessing credibility."
From: Credibility and trust of information in online environments: The use of cognitive heuristics, (Miriam J.MetzgerAndrew J.Flanagin)
So a good looking website, with content that purports to be endorsed by known authorities, and hits the right cultural blind spots for its audience will get past their filters.
Which is getting closer to the problem as I see it - the tech infrastructure has out scaled the default biological tools we are born with.
How does this work? Specifically the faster than a normal humans BS filter.