This is just a recommendation sir. I think you need another hardware or appliance that is capable of bandwidth managing. Maybe, not all of your clients/employees is not really doing “the real job” and consuming most of your network bandwidth. So, with this, if you already had an existing device which has a bandwidth manager, perhaps you can limit those clients in the lower department and prioritize those who are using critical working applications.
if you have problem in your WAN network, maybe you can purchase WAN optimizing appliance like Riverbed or BlueCoat which can boost your network speed.