I don't doubt C++ is faster than JS, but JS in modern browsers is more than "fast enough" no matter how much faster C++ may be. If you're doing the kind of things most web pages do (render some UI, information and inputs, adjust UI, take responses from those inputs, validate and send) then JS is very, very rarely a bottleneck in performance.

What JS engine does QML use? Because if it's a relatively modern one (V8, JavaScriptCore etc) and you're experiencing huge slowdowns just creating and modifying UI components then I'd seriously question the JS code you're running.