True, but in my experience bad UI response times are more related to something goddamned dumb like running a series of super expensive scroll events or watching

True, but in my experience bad UI response times are more related to something goddamned dumb like running a series of super expensive scroll events or watching DOM changes for some idiotic reason, more than about raw processing power. A C solution might make the former faster, but it'd still be a dumb architecture. A cheaper solution than migrating your whole codebase to a separate language and build process is knowing how to deal with events and how to parallelize (or delegate) stuff correctly.