If you disable all js, there is no delay. If you specifically block amp js, there is a delay. There could certainly be a shady reason for this, but the justification makes sense: one of the goals of AMP is to have, basically, a single, correct, initial paint. That's why images are statically sized and amp sticks placeholders there until the real images load. The js delay exists to allow the site to fetch the js and use it to correctly render the initial paint, even on a slow/flaky connection where the js takes a bit to fetch.
If you tell the browser to disable execution of all <script> tags, it will instead evaluate the <noscript> tag which contains some additional CSS that disables the animation.
If you disable all js, there is no delay. If you specifically block amp js, there is a delay. There could certainly be a shady reason for this, but the justification makes sense: one of the goals of AMP is to have, basically, a single, correct, initial paint. That's why images are statically sized and amp sticks placeholders there until the real images load. The js delay exists to allow the site to fetch the js and use it to correctly render the initial paint, even on a slow/flaky connection where the js takes a bit to fetch.