We don’t use REvolution slider. Please let know the best resolution size (original) to use on homepage full width just as image slider like on http://www.dermastore.ch and not REvolution Slider.
1. To us Image Gallery of Visual Composer or your Xtemos to speed up page load
2. the exact max image size to use full width like homepage http://www.dermastore.ch for best performance on page load
Elements do not influence the speed of loading. Image sizes, server speed do. Try both elements and test your page for speed loading to decide which one the best for you.
Images will be scaled. The bigger size will provide better quality. It depends on devices if you think your images will be viewed on full HD upload bigger size.