Jon Kuperman is a US developer (formerly at Cloudflare and Twitter) and a long-running Frontend Masters instructor focused on the JavaScript performance and tooling side of the modern web. His material covers compilers, bundlers, browser internals, and the engineering work that connects them.
His CourseFlix listing carries two Jon Kuperman courses on the JavaScript ecosystem and modern front-end tooling. Material is paid and aimed at intermediate-and-up front-end developers who want to understand the layers underneath their framework of choice.