Based on D8's core theme 'Stable', our new agency website http://www.internet.ltd.uk is designed to demonstrate presentation quality and speed/performance aspects of the new system. We achieved a Pagespeed result of 90 by careful optimisation and coding. We also used css flexbox to manage design elements.
The Debug tool helped greatly with the identification and customisation of twig templates.
The Responsive images styles (core) allows the browser to determine the optimum image to serve, based on the browser's own determination screen size, resolution, connection speed etc. Image styles vary from 320px up to 4000px.
This was an interesting project and worth the learning curve. We think it is unlikely now we will return to D7 unless absolutely necessary. Congratulations to all the developers and others who have created this system.