Delay in automatic creation of standard purchase orders that are created from expense requisitions

Hello,

At our project, the users create expense purchase requisitions through core forms. These purchase requisitions after some time automatically get converted into a standard purchase order.

I understand that there is a workflow that facilitates such automatic creation of standard purchase orders. However, recently the users are experiencing delay in creation of such standard purchase orders.

So I want to know if this workflow gets triggered every time an expense purchase requisition is approved? Or there is some standard concurrent program that is scheduled; so that when it runs, it selects all the available requisitions and converts them into purchase orders?