In-browser webcam demo from the polyfill author: [2]
Making this a paid-per-scan and online-required library will have a hard time IMHO.
[0]: https://caniuse.com/mdn-api_barcodedetector
[1]: https://github.com/undecaf/barcode-detector-polyfill#readme
Edit: Just updated my post with the codepen from the polyfill author: https://codepen.io/undecaf/pen/LYzXXzg
It is butter smooth with EAN barcode when using the builtin webcam on my Macbook (720p on a 2019 MBP16)
If you're looking to only scan QR codes, https://github.com/nimiq/qr-scanner is a library I can recommend and which I've used in a previous project.
One thing the BarcodeDetector API does not give you is a UI for your barcode scanning. Basically it's your job to acquire an image and pass it to the API. With STRICH, you just provide an HTML element and the scanner will live in there, including controls for camera selection and flashlight (only on Android, unfortunately).