Network equipment, wiring, location, firewalls, scanners, software. You only have so much bandwidth to share with all of the users and devices on your network. The aforementioned all contribute to taking away some of it. Just because you pay for the highest internet speed, doesn’t mean you are getting it at every computer. Everything needs taken into consideration, monitored and adjusted if you want to maintain your fastest speed possible.
Routers and switches need tuned and kept up to date with the latest firmware. You can't set and forget.
It is another key component which needs tending to for security and performance reasons.