Can you check whether you have selected "Automatically load the next task if the same person is assigned to it" for the message start event from process canvas? If you have this feature checked, then after submit the form in the message start event, it will be automatically redirect to the user task. The " POST Forms/Form/Submit" is for submitting the form in message start event, the "GET Forms/form/avsubmission" is for getting the form in the user task.

I assume the goal is to increase performance after submission and load up the next form as quickly as possible.  The delay you experience is not the html traffic you see in fiddler.  When you submit a form the Form Server extracts the submission data and transmits the data to the routing engine.  If the next page automatically loads then the forms server must wait for the routing engine to process the data before it loads the next page.  After routing engine is completed a signal is sent and the next page is generated.  If you don't automatically load next form then there is no delay because the forms sever doesn't wait for the routing engine signal before it loads next page.