Apologies in advance that this is a pretty long post, but I've reached the end of my knowledge with this problem 

We've been using the Deco M5 in our house (UK; detached; early-1990s; solid walls) since 2021. The "main" Deco is in the lounge, acting as the router; there are two "satellites", one in the downstairs hallway; and the other is on the upstairs landing, roughly equidistant between the bedrooms and the office/box-room at the back of the house.

ISP-wise, we were on Virgin Media until earlier this month - typically, we'd get up to about 140Mbps down "with a following wind" - but for cost and speed reasons, we have just switched to an "altnet" full-fibre ISP, with advertised speeds of up to 900Mbps "synchronous". We requested a standalone "modem" instead of a router, so the modem connects to the Deco, which then routes traffic around the house.

Long story short...

The good news: a speed test run on the Deco itself, consistently reports comfortably over 800Mbps down and up via the modem, and indeed the upload speed is often even faster than download (over 50Mbps more).

The bad news: it appears the Deco is massively throttling the speed of the data before routing it around the house.

I have done multiple speed tests at all sorts of locations: a computer connected via Ethernet to the main Deco itself, gives me no more than about 160Mbps down (and 10-20Mbps more up), and WiFi in the lounge is a bit lower, but generally around the low 100s.

In the kitchen (where I am writing this), I'm connected to the "hallway" satellite Deco. The "backhaul" to the satellites is WiFi, so I'd expect some further "fall-off" in speed - indeed, the speed test I just ran gave me 87.6Mbps down, and 112Mbps up. (I'm puzzled why I'm getting faster upload speeds throughout, but it seems to be a quirk of our ISP, and probably irrelevant here.)

Upstairs, things get much worse (probably due to solid walls and floors). In the office room (along from the "landing" satellite Deco, I'm lucky to get 30-40Mbps in both directions - I also upgraded our AV600 TP-Link powerline adapters to AV1000 ones, but I barely get more than 100Mbps out of the new units.

So... why does our Deco seem to be throttling our Internet speed by a factor of 5 or 6?

Remember how we used to get 130-140Mbps down (at best) from Virgin Media? I absent-mindedly checked the QoS section of the Deco admin pages (I stupidly enabled QoS some time ago, only to find soon after that you can't turn it off afterwards )... and found the bandwidth restrictions were set to the speeds we used to get from Virgin. I reset the limits to the new speeds we now get...

...and performance improved a tiny bit, but we still can't breach even the 200Mbps "barrier", even though the on-Deco speed test says we're getting 800+ from the modem.

It really looks like the Deco is the "pinch point" here. If it's getting 800-plus Mbps from the modem, but the best I can get from right the other side of the main Deco, is barely over 150... what's going on?

Two questions:

1. To (possibly) improve performance with the two Deco "satellites" (esp. upstairs): is it possible to use powerline Ethernet adapters, to provide Ethernet "backhaul" for Deco M5 units?
2. I really don't want to do a factory-reset on the Deco system... but could this be the only way to "start from scratch" and get the Deco fully working with the new fast connection? (At least a reset would probably get rid of the infernal QoS feature...)

Or... we can't really afford to replace our Deco system, but is the speed limitation "baked into" the M5, and would we need a new system to be up to the task of handling 800-900Mbps speeds?

Sorry this was such a long post - I'm at the end of what I can work out here... thank you in advance for any help you can give