Why is my Cox Gigabit Service CRAWLING???

Your signal levels look great. Looks like you have the recent upgrade to two downstream OFDM channels and one upstream OFDM channel. That is why both lights are blue.

So the speeds from the modem to router is 850Mbps but the speeds the devices are lower. Have you tried upgrading the firmware of the router? Also, what model is it exactly? GA02434-US?

FYI, if you want an ACCURATE Speedtest, test with a computer connected direct to the 8200, using a cat5e or cat 6 cable for a 1000baseTX connection, and run the speedtest. If that looks good, you have no further than the wifi to look at. BTW, I am on preferred, 250 mbps internet and using a single DG2460 downstairs, and a TPLING powerline adapter to the upstairs WiFi mesh, and my average internet speed is easily 250 on ethernet, and faster, while WiFi is 237 of 5ghz and 220 on 2.4 ghz on most of my equipment. The SLOWEST I get is 80 mbps on 2.4 on my upstairs TV, which is on a room across the hall from the mesh powerline adapter, with both doors closed. 

Looks like my previous post went into the bitbucket. My configuration is much like yours, only an MB8600 on the front. Testing directly from laptop to the back of the modem, or via the primary Google router (using either Google Home or Google WiFi apps) typically show about 550 - 580. I'm on the 500 mbps plan. My signal goes from the primary router to a gigabit switch and I use an ethernet connection to each of the 4 remaining Google routers. If you have the Google WiFi app, you can run the full set of internet/mesh/wifi tests in series. More importantly, when the 'Test Mesh' function completes, hit the ellipsis in the top right corner to see details. That will tell you exactly how fast your secondaries are talking to the primary. Mine are routinely in the 500 mbps range. The Lenovo desktop I am writing this on is using wifi and hits 400 mbps +. My wife's laptop is on ethernet and will run about 500 mbps. Two things come to mind. With multiple nodes, you can have extra challenges with a device grabbing a connection and staying on it even if the device moves. My wife's Samsung tablet is notorious for that. She'll start it in her bedroom, it will connect on a 5ghz, and then bring it down in the basement where the 5ghz connection is weak, but it won't readily reconnect. She has to turn wifi off and back on to get it to transfer to a more appropriate node. The other question is which way are your Google nodes connected? If they are not using ethernet backhaul, is there any possibility one or more have a bad connection, and/or they are functionin in a "daisy-chain" fashion rather than "hub and spoke"? The Google WiFi app will give you good insight. If you can't run in, then I'd take a laptop or other reasonably portable device with an ethernet connection and test it hardwired 1) To the Arris SB8200 2) To the Primary Google 3/4/5/6 ) To each remaining Google router. OH, and are you using first gen or second gen Google routers? I should have asked right up front. Second Gen don't support ethernet backhaul. That makes network configuration more tricky, but the approach outlined above should help you sort it out. If you post back with more detail, I can offer more suggestions. If you are on Gen 2, you may want to put all of the routers in line of sight and check performance and ensure baseline performance is appropriate. After that, work on physical layout in the house, which may require using WiFi Analyzer or WiFiman or something of that sort to ensure each secondary router establishes a proper mesh connections.