Are you looking at data passed between the Hubitat and router or between Hubitat to the Internet through the router?
Are your IP cameras integrated with Hubitat?
Hubitat is designed so that it can operate without an Internet connection. However, that can only occur if you do not use cloud integrations. If you do use any cloud integrations, there will be traffic between the hub and the Internet on a frequent basis.
For my system, the cloud based apps are Echo Speaks, ActionTiles, Amazon Echo skill and Tp-Link Kasa integration. A full 52% of the processing time of my hub is spent sending information to the Echo Speaks server. Another 33% is used by Actiontiles. Those two apps consume 85% of the processing time of the system. I have not looked at my router to see how much data is exchanged with the Internet, but I am sure it is significant.
The Hue bridge integration is able to function without an Internet connection, but it does exchange a significant amount of data with Hubitat over the local network. In contrast, the Lutron integration uses Telnet. Thus, the exchanges between the Lutron bridge and Hubitat are very efficient.
You can get an idea of which apps are using system resources by going to the menu and selecting Runtime Stats which is near the bottom of the list. The stats do not show the data exchanged, but the stats will tell you which devices and apps spend time communicating with the hub. I suspect you will find that you have some cloud integrations that are responsible for a large portion of your hub processing time. Those same apps will be the ones communicating data over the Internet.