Some developer may be still using ZXing to add barcode scanning function in their app. However, ML Kit on Android also provide barcode scanning function. And more important is, it is powered by Machine Learning. Making barcode scanning faster.
Users usually don't expect they have to queue in an app. If queueing is a must, please use Notification to tell your how long they have to wait. So they can switch to other apps. Otherwise, should the user keep this page active?