@BrooklynCoffee 

Hey

Sadly you may not like the answer to this question 

The model of Deco you are using is classified as AC WiFi (WiFi5) and yes it can connect at 866mbps, but sadly that is the theoretical maximum.   Real transfer speeds are usually around 40 to 50% of what is claimed.  Think if it as your car manufacturers claim for MPG, you wont actually see that.

This is not something that TP Link specific, its simply the nature of WiFi and affects all vendors.      Just by the nature of wireless communication its affected by interference, reflection, contention, errors, retries, encryption etc etc and all these add extra overhead to the network, which in turn reduces speed.

in fairness 250-350 is a bit low and you may find running the "Network Optimiser" may help, this will change channels to avoid interference and could bring your speeds up.

On an AC connection like you have expect 450mbps to be the max you will get, its just the way it is.   If you want to get anything close to 1gbps on your WiFi you will need to look at the AX range (WiFi6) known as Deco X.  These are the latest version of WiFi standard and has speeds of upto 3gbps, again in reality this is around 1200 to 1600mbps.   As with all new technology, these come with a price premium and of course will require your client devices to be WiFI6 capable to get those speeds.

The best place to start for you is to run network optimiser and see how that goes, if you can get it ~400 thats a good place to be.

Hope that helps and any questions just reply.

  @Philbert 

Thanks for the quick reply! My brother has an X50 so will see if I can borrow this to test and see if I get any improvement. 
 

  @BrooklynCoffee 

If your devices are WiFi6, then my guess is you will see a difference.

In honesty the AC range is only really good for up to 500mbps internet, once you approach 1gbps or more its WiFi6 territory, even then that is close to the max speeds over WiFi

  @Philbert 

Tried 3 X50s

All connected via Ethernet too each other. 
 

Connected a MacBook Pro via Ethernet - 900mbs. (Better)

via Wifi with an iPhone 13 Pro which has wifi 6, 400mbps.

QOS set to 1000 for up and down. Again no high priority devices. 

  @BrooklynCoffee 

Hey

If you are only seeing 400ish on the wifi try running the network optimiser, it could simply be you live in a rather congested area and another channel might help.  Also check that the phone is connected at full AX speeds (should be 1200), if its 866 its connected at AC.   Sadly iphones are not great for testing wifi speeds, you would be better with a laptop if honest!

Also check the cable speed on the 2nd node, plug into it and run a test.   If this is faster then its not the wifi backhaul between the nodes, so likely the client.  Remember the 2nd and 3rd node have wifi connections to them so it does use some overhead there.  Cabling into them will show you the true backhaul speeds

lastly dont set QOS, turn it off.    QOS isnt really needed at 1gbps speeds and will only mess with your testing, rule that out.

Thanks  @Philbert tried playing around a bit more.

QOS off.

Tested on multiple devices with wifi, all maxing out around 350/400. All support Wifi 6 too, did double check.

I've plugged the laptop into all the connections between deco and all points getting 900mb download. 

All devices plugged in manage to get the 900mb download. So only the wifi that's impacted.

Ran network optimaztion and that came back 'perfect'. 

  @BrooklynCoffee 

Hey

When you are plugging into the Deco nodes and testing, are the deco nodes connected wirelessly or via cable to eachother?

If wirelessly then your wifi is capable of 900 over wifi

  @Philbert 

I've tried both. Having them wirelessly connected gets me 330 downloads.

my set up is,

OpenReach Modem > Deco X50 > switch > to each Deco X50.

if I connect them via Ethernet to each as deco so they have Ethernet backbone download goes up to 417.

  @BrooklynCoffee 

Have you tried other devices on the WiFi or only the iphone?

Speaking honestly iphones are never great for this type of testing, they favor battery over speed and wont give you a real indication as it fights to reduce / lower power the wifi card.  If you have a laptop or macbook with a WiFi card this might give you a different result, just make sure you power cable it for max performance. 

I have seen this dozens of times that iphone give slower than expected results for this type of testing and shouldnt be relied on.  

  @Philbert 

yeah, tried with a MacBook Pro same speed.