Entering Sales Orders

It is taking us an extraordinary time to enter orders. For a large order
(15-20 lines) it can take up to an hour. Most of this time is spent
watching the system search through the customers invoices presumably to
check whether the customer is within their credit terms. We have been live
for only a month and are worried that this is going to take even longer as
our records increase. This only happens when we "make direct". We do not
have the calculate open order credit button checked for our customers.

Is there some setup function we are missing?