Getting 1/4 of Gigabit Speeds

I have a modest IT background and recently installed my home with a Velop router system. I have gigabit service and have the Arris NVG599 gateway. I have conducted speed tests on my Velop system which measures the raw speed from the gateway (606 Mbps at best; <550 at worst) and with the original router that came when we got U-Verse (same speed). The best signal I have ever gotten wirelessly on the Velop is 340 Mbps when my AC device is literally inches from the actual router. How can I (a) get as close to gigabit as possible wired and (b) provision my router to broadcast as close to ~80% of that signal as possible? 

Also, I do not have any other devices hardwired into the gateway and the wireless speed will often drop its max to sub 200 Mbps after a few minutes.

Thanks!

P.S. I have already restarted the gateway, put the gateway in bridge mode (made no difference), and factory reset the gateway.