@Kevin_Z 

1. I did that, I got similar speeds 630-700 down, 40-43 up (hardwire gigabit connection)

2. I tried connecting a different computer via wifi, I got 400-550 down, 40-42 up

3. There was nothing else running at the time. I tried it from a Windows computer and a Mac

@Newcastilian 

Thanks very much for all the detailed info you've provided now.

We would like to escalate your case to the engineers who will help you fix it. Please check your email box and respond.

Good days.

@TomDumoI am having the same issue. I have tried all the things that were posted to do. So I hooked up my AC5400 router and my speed almost trippeld. Since I hooked up my AX11000 router my speed lost almost 2/3's. Now I wish I would of never purchased this router. $400.00 down the drain. Does anyone have a solution yet to this problem?

@Phxkid, as some other mentioned before, I also noticed that activating QoS resulted in reduced speed.
Deactivating this fixed it for me.
Since then, no issues at all anymore and very happy about this router.

Hope you get your issues solved soon!