Javascript being interpreted is not a big deal for most UI (it's not like your code has to be in JS, it just needs to interop with it for the UI) and JS has been optimized more than most languages (eg it'll run circles around "desktop" languages like Python that don't ffi to C libraries).

The problem is the non-nativeness of it more than anything else. It's also what brings the bloat and the slowdowns because everything is reimplemented from scratch.