Yes, AMP can cause some pages/content to load more slowly. This was stated in the write-up, and supported a link to Google's own DOJ disclosures. In that source we read that Google knew that some publishers avoided AMP because their own pages were shown to load more quickly without it, especially considering 1-second throttling on Google's part for non-AMP pages, aimed to give AMP a "nice comparative boost".
AMP loads instantly. There is nothing lazy about supporting it. Just like RSS, it requires extra work for the publisher to get the user the instant-loading behavior they desire.