When hitting Create Invoices and Charge Jobs at the bottom of the I&J Report multiple API calls will be made for each job. Depending on how many jobs you have this process can appear to be slow, but it is just the magnitude of things going on in the background.
Each Job does the following:
API Call - QBO Get Customer
API Call - QBO Create Invoice
Database Call - Save QBO Invoice Id
Database Call - Create Additional Pay for Tips
API Call - Auth.Net/Stripe Charge Card
API Call - QBO Create Payment
There are a few one time calls that happen also to retrieve QBO Items/Accounts, get Jobs, get Time Sheets, etc.