Check-out takes more than 2 seconds per item

Overview: Check-out of multiple items is too slow.

How to reproduce:
Open http://folio-snapshot-stable.aws.indexdata.com/checkout
Select a patron, the patron data appears in the left pane.
Enter the barcode of a the item to loan.
Press the enter key or press the "Enter" button.
Now wait until a message about successful check-out appears and the item barcode field gets wiped out for the next item.
Expected response time: Less than a second (or even faster).
Actual response time: More than 2 seconds, see attached screenshot.

If you enter the next item's barcode without waiting it gets wiped out. The "Enter" button and the enter key are disabled during wait time.

Check-out of 10 items to the same patron currently takes more than 20 seconds, it should be less than 10 seconds (or even faster).