Why is my internet slow? (With a good router, modem, and advertised speeds)

Hello,

I'm trying to figure out why I have random drops of internet speeds when I'm watching Twitch, Netflix, or just loading up a web page. Sometimes I'll have blazing fast speeds, other times it takes almost 30 seconds for google.com to load a search. Below are my settings and home
configuration.

ISP: Comcast

SpeedTest.net Results: ~30 Mpbs down; ~7 Mbps up; 11ms ping

Router: NetGear Nighthawk R7000
Some Settings:
WMM Enabled (WiFi - Multimedia)
QoS upstream and downstream disabled
UPnP Enabled
Dynamic DNS enabled
IGMU Proxying disabled
MTU at 1500 bytes (tested this from tutorials and this should be optimal)



Modem: Motorola ARRIS SURFboard S86121

Affected Devices: Everything, from 2 PC's with Windows 10 and Android Tablets, to Mac laptops and iPhones. (on both 2.4GHz b/g/n and 5GHz a/n/ac bands)

If you need any more info, I'll be glad to give it!
Thanks for the help!