Help to diagnose wifi issue

[#1]

We just upgraded to full fibre and switched from BT to Plusnet on their Full Fibre 900 plan. It activated today and for the most part is meeting the expected speeds on devices around the house, except my PC, which is one floor up and almost directly above the router. 

Everything else is getting around 500-600Mbps, but my PC only reaches ~250Mbps before it caps out, and I can't figure out why.
The PC uses onboard Wifi and has two antennas on the back. I've tried rotating the PC and adjusting antennas for optimal positioning with the router, I've tried 2.4Ghz on various channels (caps out at around 40Mbps), and 5Ghz on various channels, but 250Mbps is the best I can get.

I've updated my Intel WiFI adapter drivers but this had no noticeable effect.

The WiFi adapter on the motherboard is WiFi 6 but we're using 5 because the router is, I think?

Wifiman reports a good signal in this room (-35dBm to the "Arcadyan" access point/router), no meaningful packet loss, but latencies do go into the hundreds when downloading a large test file.

Any advice?

[#2]

Has your PC ever operated at more that 250Mbps on 5 GHz Wi-Fi in its current position ?

Or is it the case that you have changed from a slower BT FTTC service to Plusnet FTTP 900 service
and are only now able to see real limitations of the PC's 5GHz Wi-Fi in your environment ?  

[#3]

@hindmost 

Is there any possibility of getting someone in to run an external ethernet cable up the wall terminated in wall sockets at either end?

This provides a solid starting point for internet upstairs.

Brian

[#4]

We were capped at 74 with BT so I never had the opportunity to test higher speeds. 

I am looking at having someone install an ethernet socket as an alternative but I've heard that proximity to power lines will cause interference. Is that an issue?

[#5]

Ethernet cables are fine for up to 100 meters in the maze of 3 phase power cables found in a high rise offices.

At home I would not be concerned about in house mains power lines.

[#6]

@hindmost 

If you're paronoid you could specify external grade "shielded" ethernet cable but I hate to think how much that would cost!!!

If you get wall sockets you can also get "angled" ethernet plugs - the cable points down (or up, left or right) so it is not sticking out.

Brian

[#7]

Tip, only use shielded Ethernet cables where the devices have grounded metal sockets.
Plastic network sockets would leave the shield or screen floating and it then becomes and antennae !

If you are going to have network cable run to the office with the PC, also plan to have a Wi-Fi Router ( in access Point mode ) in the office so you end up with spare gigabit network sockets & better Wi-Fi coverage for the rooms upstairs.   A device costing less that £50 might be all that is needed.

[#8]

Good tip, thanks!

Any suggestions for a router/access point?

[#9]

Pick a device with Gigabit network ports.