Nighthawk MK63 Speed issues

What Firmware version is currently loaded?
What is the Mfr and model# of the Internet Service Providers modem/ONT the NG router is connected too?
Be sure your using a good quality LAN cable between the modem and router. CAT6 is recommended. 

What is the size of your home? Sq Ft?

What is the distance between the router and 📡 satellite(s)? 30 feet is recommended in between MR📡 and MS🛰️ to begin with depending upon building materials when wired or wirelessly connected.
https://kb.netgear.com/000061709/Where-should-I-place-the-Mesh-WiFi-satellite-of-my-Nighthawk-Mesh-WiFi-6-and-Tri-band-Mesh-WiFi-6-products 📡 ‌‌🛰

What channel configurations are currently set on the router?

What wifi devices are you speed testing with? 

Turn OFF the MS and speed test with the MR only. 

Speed test with a ethernet connected PC with the MR.

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SudsNighthawk wrote:
.....I ran a separate SpeedTest from my phones and laptops, and the max speed I get is ~150mbps. Any advice for what I can do?

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That test depends on the capabilities of the wifi clients you are using. That 150 Mbps speed is what you can expect from some devices, especially if they are on 2.4 GHz.

So, we need more details to see if your problem is with the router, the wifi clients, or something else.

Also check that the phones and laptop get much better speeds elsewhere.

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I have a 1000mbps fiber connection and the Netgear Nighthawk shows a speed of ~900mbps to Modem.

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Spot on there. In effect, that test is identical to using a wired connection from a PC with an Ethernet connection.

The speedtest in the app bypasses the wifi. It just tells the router to test the wired speed between it and the modem. Running it from the satellites should deliver the same result. The wifi link just starts the test and tells you the result.

I mention that only because people get confused about what those tests are doing. But you knew that anyway.