The pages load fast, they are just styled to not be visible until after 8 seconds.
For example, using the site used in the article, you can block 3rd-party scripts and bypass the AMP CSS using the following cosmetic filter in uBlock:
scientias.nl##body:style(animation: none !important;)
When I found out about this, I tried to find the reasons for this artificial "delay" in the AMP documentation: I can't find any valid reasons for the artificial delay.
The net result unfortunately is that most users wanting to block `ampproject.org` out of privacy concerns are going to feel the need to whitelist `ampproject.org` to "un-break" a site making use of it.