There are several reasons why the nutrition label scanner is taking longer than 10-15+ seconds.
First, make sure that the label is clearly legible by the camera:
If you've tried all of these steps and the label still won't scan quickly, it's likely that there is something about the label's design that is causing issues.
Some nutrition label designs perform better than others. Designs that have these characteristics tend to scan faster and more reliably:
In practice, labels from the United States, Canada, and Mexico tend to perform very well, as these countries have standardized on label formats that exhibit these properties.
In addition, newer devices with faster processing capabilities (e.g. iPhone X or later) will be able to scan labels more-quickly.