WNDR3800 Slower Speeds than Modem

When testing my download speeds directly from the modem, I get around 80mbps, and when I go through the router (WNDR3800) I get around 30mbps. I've tested this on each of my devices (both wifi and ethernet) while all other devices were disconnected and they all gave a consistent 30mbps. I've made sure I don't have any QoS settings on as most places online contribute that as the main factor to slower speeds on devices. Can't really find anything else online as to what the problem is so any help is appreciated 🙂