I used Google PageSpeed Insights to measure the speed of the themes, using the demo url, included in this list.
Schema
Schema is one of the fastest loading, ultra-SEO friendly WordPress themes. Featuring all the best MyThemeShop features, including custom options panel, tons of shortcodes and widgets, and a pixel perfect design, Schema also includes rich snippets in order to help search engines your site and rank you higher.
Google Pagespeed Insight score: 89
Demo/Download
Avada
Avada is the top selling theme of all times on ThemeForest. Besides its impressive speed, the theme also features a builder that lets you to personalize your site with no coding knowledge required.
Google Pagespeed Insight score: 89
Demo/Download
Nexus
Nexus is a Fully Responsive, Modern and Multipurpose Template based on Bootstrap 3 framework. It is fully responsive and looks stunning on all screen types: desktop, notebook, tablet and smartphones.
Google Pagespeed Insight score: 89
Demo/Download
Bootstrap Basic
Bootstrap Basic is a free theme that can be used as a starting point for building your own theme. Great speed score and solid code makes it a great foundation for anyone looking to create a custom WP theme.
Google Pagespeed Insight score: 87
Demo/Download
BeTheme
BeTheme is a very popular theme that has been discussed in this post a few weeks ago. For those already interested in it, you’ll be happy to hear that BeTheme is scoring 87 on Google Pagespeed Insight, which guarantees a fast loading of your site and most chances to get a good ranking on Google.
Google Pagespeed Insight score: 87
Demo/Download
Impreza
Packed with tons of features and add-ons, Impreza is all you need to kickstart your new blog or website.
Google Pagespeed Insight score: 87
Demo/Download
Divi
Brought to you by Elegant Themes, Divi is the ultimate Multi-Purpose theme fit for just about any occasion and powered by the Divi Builder, which allows you to create and customize websites quickly and efficiently.
Google Pagespeed Insight score: 84
Demo/Download
Enfold
Enfold is a clean, super flexible and fully responsive WordPress Theme (try resizing your browser), suited for business websites, shop websites, and users who want to showcase their work with a neat portfolio site. The Theme is built on top of the fabulous Avia Framework and offers support for the WPML Multi-language plugin.
Google Pagespeed Insight score: 81
Demo/Download