Yes, its basically a shortcut when I am doing a quick first scan through resumes. The moment I see one of those keywords I will flag it for 2nd pass review and move on.
As long as it makes sense when you read it after a few days (i.e. its well organized) and it also works as expected, then you are probably doing well.
Javascript is actually incredibly fast on most devices, especially if you are constraining yourself to the vanilla API and not something like jQuery. Remember, every web browser's JS engine has been hyper-optimized to support decades of half-assed website implementations.
It's really hard to screw up perf on document.getElementById(). There's honestly not a whole lot of ways to hang yourself with the vanilla methods unless you are trying to build something ridiculous like a raytracer or physics engine.
i think the biggest thing for your average app is just causing a lot of layout repaints. as long as you spam requestanimationframe everywhere and group style changes and polling, you'll be good. I was really surprised how good I was able to get things on even an ancient ipad 2 I had laying around