@InfiniteRiddle 

As I said previously there are several factors that could be at play here causing the poor performance.  We would of course be happy to assist in troubleshooting this with you. First a side note.  As I mentioned to @MikePCTECH  there was a known issue with Verizon FIOS on some of our WiFi 6/6E Decos.  Thankfully they have confirmed that is not something that affected the Deco M4.  Though with you having Spectrum that was less of a factor, still wanted to reassure you that it would not be a factor here. 

Now for the troubleshooting.  Let's start with the modem.  What you would want to do first is log into the modem.  This is typically done from any browser and typing 192.168.100.1.  Once in your modem, you will want to check the logs.   Specifically, you are looking for T3 or T4 timeouts.  These indicate an issue on the line or the CMTS card and will require an ISP maintenance dispatch to the plant.  Next, you will want to check the connection status.  Make sure all channels show "locked".  Locked means active. Next, you will want to look at the uncorrected errors.  There typically is a few but if there is a large amount you will want to ask the ISP for a tech dispatch.  Look at the power levels.  Each Downstream channel should be between -12 and +12 and the SNR should be below 55.  If any of the channels are not in this range or below 55 you again will want to ask for a house call from the ISP.

Now, say that the modem and levels are all good and there are no T3 or T4.  We can then safely rule out the modem or the ISP service as the choke point.  So let's now focus on the router.  First, make sure the ethernet connecting the modem to the main Deco is at least Cat5E and is firmly connected in both the modem and the main Deco.  On the Deco as you may know it doesn't matter which port is used as they are auto-sensing.  You will then want to make sure the Deco system is fully updated by checking for new firmware in the Deco App.  Now if the Deco is connected directly to the modem and it's not a gateway (modem/router combo) then Deco will need to be in router mode.  This is where the troubleshooting advice stops for now and the questions begin.

1. Are you using QOS?
2. If so did you set it up according to this information?
3. How are the Decos backhauled?  Are they linking to each other wirelessly or through ethernet?
4. How far apart are the Decos? 
5. When testing speed are you testing with devices physically connected to the Deco via Ethernet or wirelessly
6. If both what were the speed differences? 
7. Wireless client testing, What node is your client connected to and how far from that node is the client?

We will end there to make sure the response isn't too long and information doesn't get lost. 

  @InfiniteRiddle 

You don't want to skip the modem steps, even if you believe it to be redundant or unnecessary.  Just because one router is working near your maximum connection speeds doesn't mean there isn't something there.  You still check that box.  Especially since each router can build out a different path to the same destination and at times build a "dirty" path with packet loss and congestion that will slow the performance of the product.  Additionally, the connection to the house can be good but from the pole to the plant or the CTMS card could have issues.

Cat6 cable is good and it confirms that getting the speed of your connection is possible.  We also want to make sure that it is firmly connected to the router and to the modem.  Ethernet backhaul is actually much better than wireless and is recommended if your home is wired for ethernet.  Transmission over ethernet is cleaner, more stable, less congested, and generally faster than wireless.  

Your Desktop when connected wirelessly what unit was it connected to?  You can tell in the Deco app by tapping on the Deco Map and then seeing which unit has the client connected to it.  Was this Deco the main or a satellite unit?  What was the link rate the PC was connected to the network?  Can you post the speeds up and down from a site like SpeedTest.net?  Please run the test 3 times on the same server to get an average speed. Same Question and test from Wired to the unit. 

You can also try changing the DNS servers to something like Google's or OpenDNS as they generally perform better and you see better speeds from these rather than the ISP's