RAX45 / AX4300 speed test run from INSIDE router shows 50% expected speed.

its not a firmware issue, its an issue of the integrated speedtest in the app.

Problem with it is that it chooses the closest server, not the fastest server. so we see frequently that people don't think they're getting the full speeds because the built in speedtest through netgear doesn't show it. Tends to happen a lot with people on gigabit because its a challenge to fully saturate. 

The best way is the hardwired test using the actual speedtest in the browser but even that sometimes chooses a poor server. so we usually recommend trying the speedtest site, dslreports, and even google/xfinity's speedtests just to give you a rough idea.