@Don_Gage 

This resolved the same Issue I had on the AX6000 with slow LAN speeds.

Direct to modem speed was 500mb+

While connected to AX6000  -  122MB

Cleared the device priority, and set to standard fixed it.

Now getting 498mb through the AX6000.

Didn't change the AV setting on router.

@Tony 

WOW! from 20mb to 59.

Thank you Tony!

@Don_Gage 

You must also disable (i.e., uncheck) Link Aggregation then save the change (which requires the router to restart).

The Link Aggreagation setting is under Network (Advanced) > LAN.

Don_Gage wrote

I have a gigabit internet connection with Spectrum.  When connecting the AX6000 to my ISPs modem the LAN speed has never gotten past 500mbs on Speed Tests conducted using the built in Speed Test.  I've also tried testing the Download speed using my Computer while connected to the AX6000, getting the same results.  Thinking it was related to maybe my ISP throttling my connection I then tried hooking my CPU up directly to the modem and obtained a speed of 993mbs.  Why is the AX6000 cutting my connection in half?  What setting needs to be set or unset to allow me to see a better download speed? By the way when conducting these tests, my computer was the only Lan connection.

Tried the above and no improvement - Spectrum @Modem is 400+Mbs down 20+up, after changes still no improvement with speeds through the AX6000?   

@Tony ?