That's interesting. We actually use quaggaJS quite heavily, and haven't found any large performance issues even on lower spec devices.
That being said it did take quite a lot of time to get to that point (tuning scan frequency, resolution, and a bunch of other things), and we looked into scandit recently (our company does use scandit in other products) and even though your library can detect and scan barcodes in worse conditions, it didn't really improve scan times or fix our biggest issues on web, which by the looks of it are the same issues you are running into. (iOS 11 being a pain in WebViews and sites added to the homescreen, and lack of the focus management APIs meaning you are at the mercy of the autofocus), plus the requirement for a license check at runtime is shooting our big use case in the foot (some of our users run the webapp offline for days at a time!)
But we have looked into some webassembly for some of this code, and I'm glad to hear it holds up about as well as expected on lower spec devices. I was worried about lower spec devices not being able to handle the larger "binaries" that WASM tends to produce.
And this is really off topic, but I'd love to pick your brain on how you "solved" the constraints issues on some devices with the getUserMedia stuff (flipping the aspect ratio requested, some devices rejecting all constraints except 2 or 3 "blessed" resolutions, problems with orientation affecting the returned resolution regardless of what was asked, etc...). I understand if you can't talk about it due to company secrets, but I figured I'd give it a shot! If you are open to it, my email is on my user page.