This rather new form builder, with at the time of writing only around 500+ installations, seems very promising. Built with HTML5, performance and accessibility in mind, WS Form Lite is definitely worth a closer look. The drag and drop building implementation is the best I’ve seen so far amongst form builders, the premium feature ads are subtle, framework detection a unique feature as far as I know, and the performance is incredible. Tiny CSS files.
Extract from WordPress.org
WS Form was built with enterprise level performance in mind. WS Form does not interfere with the loading of other page or post content on your website and isolates submission records to ensure as your website grows your page load speed is not affected. All form code is HTML 5 compliant and if your website theme uses a framework such as Bootstrap or Foundation WS Form automatically outputs code to suit that framework. This means your form loads as fast as possible without any unnecessary overhead typically found in other WordPress form plugins.https://wordpress.org/plugins/ws-form/