When you mail in a return with payment, the payment is taken to be processed in a separate queue.
Imagine having a business that gets thousands of payments back; you'd rip them open and get all the checks and hand them over accounts receivable to take to a bank or EFT them. That would be the quick part.
Then processing the returns, as they involve data entry, computer matching to other forms (e.g., W-2s sent in by employers, 1099s by banks, etc) and since that takes longer, it's a backlog.
The check is the rabbit, the return is the turtle.
