Autoptimize; because Optimizing Matters
With over 1 million active installations, Autoptimize is one of the most successful free and open source WordPress performance-centric plugins. The plugin, as well as e.g. Async Javascript and WP YouTube Lyte, is developed and supported by Optimizing Matters (Frank Goossens, Belgium). The company also builds partnerships with 3rd party services (e.g. Shortpixel & Critical CSS) and provides WordPress performance optimization services on https://autoptimize.com/.