CS4070 scanning speed is slow down

Regarding SYMBOL CS4070,I have question.

We are developing web application with PHP/javascript.Browser is chrome in iPad/CS4070(bluetooth HID).
The application read the QR code continuously for check the inventory at the end of the month.
At first, scanning speed is normal, but as the number of reads increases, scanning speed is slow down.
Why.
If you have any information, please let me know.

*On the other hand, there is no problem with the configuration of Chrome/Windows10/CS4070(bluetooth HID).