Slow Local Wifi/Fast Wired Internet RBK852

Wifi speeds won't be as fast as wired speeds. There's a lot of reasons but mainly it comes down to network overhead, wireless interference, obstructions, and device limitations. 

For example, the 8 series (your router) has a 1200mbps 2.4ghz and 2400mbps 5ghz fronthaul. But thats with a 4x4 antenna setup. Most devices (phones/tablets/laptops) are 1x1 or 2x2 antennas. So the best possible connection is at 1200mbps. But thats just link speed. Actual throughput tends to be around 55-65% of link speed. that'd put your around that 600-800mbps range for actual throughput. And thats if you're connected at the maximum rate and don't have much interference in your area. 

I'd take a read through duckware's article on it. Its a bit in depth but helps explain it really well. 

https://www.duckware.com/tech/wifi-in-the-us.html