Its the time it takes to run one task. It generally takes around 7 seconds. When more tasks build up on the integration bus the time it takes to run a single task increases dramatically. It goes from around 7 seconds to a couple of minutes.
It only takes around 4 tasks to build up for it to slow down.
The handler is essentially just creating a pdf and saving it to a share. When a customer creates an order the order goes through a workflow process. And this pdf part of the workflow is creating the invoice and then putting another task on the bus to then upload it to AWS.