Our latest numbers show that the average speed improvement of the AMP cache is roughly 75%. I'm a bit shocked by that myself. But that is where it stands.
Getting similar performance out of on-origin serving is a priority for us, but we aren't there yet. One challenge is to give it the same scale as the AMP cache.
Could you at least make it possible for websites to opt out of the AMP cache? Last time I checked, publishing an AMP page grants Google (and everyone else) an implicit license to incorporate your content on their own web properties. The AMP FAQ say: "Should you desire not to have your document cached, one option is to remove the amp attribute from the HTML tag. This makes the document technically invalid AMP, while not impacting the functionality of the document." But from what I understand, this will also impact the presentation and ranking in search results.
Getting similar performance out of on-origin serving is a priority for us, but we aren't there yet. One challenge is to give it the same scale as the AMP cache.