I've been playing with WebAssembly lately and the moment where it clicked for me how powerful it was was building an in-browser crossword filler (https://crossword.paulbutler.org/). I didn't write a JS version for comparison, but a lot of the speed I got out of it was from doing zero memory allocation during the backtracking process. No matter how good JS optimization gets, that sort of control is out of the question.
I also think being able to target the browser from something other than JS is a big win. 4-5 years is a long time for JS, but not a long time for language tooling; I feel like we're just getting started here.