joker97: If you are connected at 144Mbps to your router, that is one issue [either at router end or Dell end] ... may or may not explain everything.

If connected at "N", should be 300Mbps, if "AC", should be 750-850Mbps.

Can you disable 2.4GHz on your router and ensure enable WWM/QOS and 20/40/80MHz enabled on the 5GHz channel

Sideface:

joker97: If you are connected at 144Mbps to your router, that is one issue [either at router end or Dell end] ... may or may not explain everything.

If connected at "N", should be 300Mbps, if "AC", should be 750-850Mbps.

Can you disable 2.4GHz on your router and ensure enable WWM/QOS and 20/40/80MHz enabled on the 5GHz channel

As suggested, I have disabled 2.4HGz on the UniFi, and changed from 40MHz to 80MHz. (the only options are 20 or 40 or 80)

There has been an immediate improvement in performance.
The Network centre now shows more than 500Mbps (varies with location).
I can now get full line speeds on the Dell, just like the Vaio.
The Lan speed test is now above 100 Mbps:



(click to view)


As a background to all this, wireless performance was so poor on delivery that after a week I tried to install newer Dell network drivers (from the Dell website).
This was a disaster, with complete loss of wireless connectivity.
So i wiped the hard drive, reinstalled Windows 10 (easy) and all the latest Dell drivers (very difficult).

I discovered the hard way that most of the drivers would not install without dot net 3.5 - this was NOT mentioned on the Dell website.

I am still not convinced that all the network drivers re-installed correctly, although the Device Manager states that they are all installed and up-to-date.

Thank you all for your help.

Jase2985: @Sideface those speeds still dont look great for wifi AC are you sure its not connected via N on the 5ghz band?