Depends on how the telco handles tethering. If the telco allows unrestricted access, you are only bound by the wifi speed of your phone. If you have a 5s, then the iPad pro will only be able to surf at N speeds even if you are connected on an LTE-Advanced network.
Based on my experience, my telco knows that I'm tethering and throttles the speed to 1mbps (I'm on unlimited data).