However, the web page size is not the only factor impacting the site speed. Even significantly larger sites load and render quickly if they’re smart about how they deliver.

We found the top 700 UK ecommerce sites, based on their estimated monthly traffic as provided by SimilarWeb. For each, we analysed them using Google’s PageSpeed Insights API, checked their page size and loading time on Pingdom, and verified their HTTP protocol using https://http2.pro/. From this, we found the following data, and used it to determine which websites are best optimised for speed:

• PageSpeed Insights Desktop Score (not considering third party data)
• PageSpeed Insights Mobile Score (not considering third party data)
• HTTP/2
• Web page size
• Loading Time
• Loading Time per MB