There are hundreds of thousands of lines of JS making up the "browser" parts of the app known as Firefox. "Written in JavaScript" is not the thing that tends t

There are hundreds of thousands of lines of JS making up the "browser" parts of the app known as Firefox. "Written in JavaScript" is not the thing that tends to make web apps slow. It's all the terrible "best practices" associated with the NPM ecosystem that causes that. You can generally sidestep the performance problems by just not programming in that style.