Using WordPress’s built-in responsive image capabilities with Advanced Custom Fields can speed up your site.
Advanced Custom Fields changed my life. At least in terms of using WordPress as a full blown CMS instead of just a blogging platform. Sure we were able to add custom fields before, but, to a user, they were ugly and unintuitive. ACF on the other hand gives us flexibility and control to add fields images, text, WYSIWIGs, and more directly into the post page. Which keeps the client happy and cuts down on the number of times we have to explain how to change their header text. Always a plus.