[chap-embed] shortcode uses an instrinsic aspect ratio which allows it to resize responsively based on their expected aspect ratio and not by using a single specified width or height.
Including an iframe embed directly will automatically load all iframe contents on page load which can drastically reduce the responsiveness of a page.
Semantic UI's embed implementation combats this by not loading iframe content until a user has interacted with your embed's placeholder content.
To give you an idea of how much file size including an embed will add to your page load see the chart below:
<chap-embed image="http://i.imgur.com/yVzwFHB.png" icon="marker" amp-height="600" url="https://www.google.com/maps/embed?pb=!1m18!1m12!1m3!1d12418.656683985166!2d-123.70117385610186!3d38.908792273100936!2m3!1f0!2f0!3f0!3m2!1i1024!2i768!4f13.1!3m3!1m2!1s0x8093b7192ca69f27%3A0xcdc7d22a4a426ea0!2sCA-1%2C+Point+Arena%2C+CA+95468!5e0!3m2!1sen!2sus!4v1485179477978">
<chap-embed url="http://myfav.es/jack" icon="right circle arrow" image="//placehold.it/500x360" amp-iframe="true">