Whatever performance tool or test you are going to use for an AMP page, it will not reflect representative results and metrics.
Developers implementing AMP are looking to get blazingly fast loading speeds, and are therefore often curious to see how much their site improved with AMP. However, a naive run of one of the many available performance tools (e.g. Google PageSpeed Insights or Lighthouse) does sometimes yield surprising and seemingly suboptimal results.
To understand why this is happening, it is important to be aware that AMP speeds up a website on three different levels:
- The first level of improvement is that an AMP page is thought to load instantly on a mobile browser and it's already as fast as possible, as custom JS is forbidden, critical path is unblocked, CSS is inlined and many other optimizations such as a special HTML code.
- The second level of speed-up is then achieved through the caching by the Google AMP Cache, which will reoptimize images, add prefetch hints, minify html, serve via HTTP/2, along with many other optimizations. Indeed an AMP page is never served directly using the URL of your website and from your server, but on the contrary it's fetched from the Google AMP cache domain.
- The third (and potentially most impactful) level of speed improvement is based on the fact that AMP can be prerendered in a safe and secure way, by prerendering only assets in the first viewport, and not executing third-party scripts.
So if a performance check is done on AMP on origin, the speed scores (while normally much faster than the canonical) are not yet representative but misleading.