This is strictly slower than an AMP preload, which is why I mention that even a .txt file will load slower. The .txt file document still requires a network round-trip, regardless of javascript.

On some wired connections, a network round trip is within user-perceivable 'instant', but for many people on mobile devices, this is very noticeable.

https://hpbn.co/mobile-networks/

Even on the best LTE networks, the 'core network latency' is 40-50ms. 'core network latency' is the latency for getting the packet from the phone to tower to the packet gateway. This is before the packet even goes on the internet. And you need to double it for the round trip. You usually need to double the whole thing for initial DNS lookup.

Best possible latencies for an html page load on excellent LTE connections without prefetching are around 150-200ms. Most users in the world will experience >1s. A United States major city wired connection is not at all representative of what most people experience on a mobile connection.