I remember looking at the original HTML5 spec and going, there is no way lower power / low bandwidth / high latency devices will be able to handle this efficiently. Nevertheless, we moved fast and broke things, ratified the HTML5 spec and paved the cowpaths without a second thought to language or efficiency.
Somewhere along the line, things came full circle. HTML was slow again, so we needed a new new way to efficiently render content, thus AMP was born.
In what way is HTML5 is inherently slower than previous versions? Pages tend to be slow because of the tracking and adds addded to them as well as client-side "rendering" for static content. None of that is required by HTML5.
Essentially html5 is significantly harder to parse because the amount of branching that has to be done. If you go look at the amp specification and the reasons behind it, they designed it to be a stripped-down light version of HTML5.
Somewhere along the line, things came full circle. HTML was slow again, so we needed a new new way to efficiently render content, thus AMP was born.