@CMsc 

It looks as if Deco units can't keep gigabit speed through that cat6 cable and downgrade it to 100Mbps.

I would suggest the following: configure Deco mesh to run in Access Point mode and, then, connect Ethernet cable from Satellite Deco to Virgin Media router (instead of Main Deco). 

Also, if your Virgin Media router reports status of its Ethernet ports, next time it happens (if it happens with that new setup) you can check there if port speed changed from gigabit to 100Mbps.

How to set up Deco to work in Access Point mode

  @Alexandre. I had trouble running these as access points. Responsiveness seemed to drop off, and I'd get weird laggy things happening, so I just abandoned it. I'll give it another go.. 

When I just had the cat 6 cable connected directly to my computer from the virgin hub, all was fine before. No drops in speed. So is the problem that the S4's are just junk, with poor ethernet connectivity? They seem to work for a day or two at gigbit speed, before dropping to 100mb. 

  @CMsc 

Three out of four Deco I have are wired. They run in AP mode for over 2 years now. I have not had issues with speed and performance during the time I own them. My Internet link is gigabit. My Deco are M5 and M9 Plus.

  @CMsc 

I can only say that these don't work properly. 

Here's my two setups I've tried:

a) Virgin Media Superhub3 --------cat5e--------Main Deco S4----------cat6----------second Deco S4. 

b) Virgin Media Superhub3 --------cat5e--------Main Deco S4----------cat6----------gigabit ethernet switch----------second Deco S4. 

                                                                                                                                                          I

                                                                                                                                                          I

                                                                                                                                                          I--------cat5e----------Windows PC/Mac

The Decos have been tried in both router and access point mode - if anything they're less reliable in access point mode. The time it takes for the speed to ramp up to the 350mb max download speeds is random. In access point mode it sometimes takes 10 seconds for the speed to increase to maximum. In router mode it's better (or certainly feels better). 

With both set to router mode I get a snappier response for some reason. 

But no matter what I do, the second deco s4 gets limited to 100mb speeds randomly. With the gigabit switch (setup b, above) added, I can check whether the issue is with the second deco, or the main deco. And I can confirm that when the second deco drops to 100mb speeds, so does the wired connection to my Mac/PC. 

If I remove the second deco, and just run the cat6 cable direct from the Virgin Hub into the PC/Mac I get full speed all the time, all day long. 

I've ordered a new cat5e 50m cable to replace the cat6 cable with, just to see if thats the issue (obviously the deco's are the issue, but just to see if it allows them to work more reliably), but I won't know anything until next week. Other than that, they'll be getting returned to Amazon. 

  @CMsc 

Ethernet cables make the difference. I am lucky to have access to good quality professionally tested Ethernet cables. My house is wired with Cat 5E, they handle gigabit speeds well and also don't cause issues with Deco mesh.

If you can't make Deco hardware work for you, makes sense to return it. This is my advise to anyone buying networking gear: get it from place with good return policy, so that you can send it back if it does not work for you.