Rax50 Inconsistent speeds across different devices

200mbps is more than enough for gaming/streaming. Even 4k only needs 25-40mbps for streaming. 

so hitting 200mbps, you won't have issues. 

to explain the speedtest issues can take a little bit. 

First off, when you speedtest using the nighthawk app, it actually hosts the test directly off the router. So it doesn't involve the wireless variables. This can be the best speed but it can also be slower. Reason why is the built in speedtest in the nighthawk app doesn't always pick the best test server to use. So sometimes it'll max out gigabit, other times it struggles to hit 300mbps. I rarely recommend it when someone has speeds great than 500mbps. 

When you're hosting off wifi, then it involves a lot of other factors such as interference, antenna's in the device testing off, DFS channels, etc. As well as it can involve the same issue of did speedtest choose the best server to test off or not. Its why the actual speedtest app lets you choose different servers. Many factors influence this. 

the most reliable indicator is testing hardwired into the router and testing off your ISP's speedtest site. Tends to actually max out a connection and the hardwired means there isn't anything interfering with it.

From the sound of it, some of your devices are doing great on wifi with a couple being a little slower. "little slower" still hitting 200mbps. Unless its causing issues, I wouldn't worry about it to much as 200mbps is more than enough for anything but massive data transfers