@Gmoon 

Hi

Range Extender works in 'half-duplex' mode in extender mode,  it can only talk to the main router or the wireless client device in either way, not the same time, the link speed at physical layer will cut to half. Real 5Ghz speed could be around 50% of wireless link speed. 

To check if the speed is reasonable, you could refer to the thread below to troubleshooting and help provide information below: 
What should I do if Internet speed becomes slow via Range Extender?

Check the Wi-Fi link speed between range extender and client: How to check the associated/link speed on a wireless client 

Speed test result from router 5G Wi-Fi at extender's location;Speed test of extender.
 

  @Joseph-TP 

Hi Joseph,

So what you are saying is I should be getting 50% of my wired speed. Well I'm only getting 10%. Even at 50% I should be getting around 400mbps. 

Neither of your articles are any help to me. There is no reason why I should be getting 90mbps when the connected eithernet is 900-1000mbps. The devices I am connecting to the RE605X are about 30cm away and are all hitting the same speed. I think the spec TP are claiming is marketing nonsense. Unless you can offer a solution I'm returning this product.

Thanks

Gary

  @Gmoon 

Hi

The following suggestions are for your reference:
1. It is recommended that you refer to method 2 of the link to upgrade the extender.
https://www.tp-link.com/support/faq/1406/
2. It is recommended that you turn off High speed mode. You could refer to the following link to find the path to high speed mode:
https://www.tp-link.com/support/faq/2479/

90mbps does not seem like 5G speed.
Are you connected to the extender's 5G Wi-Fi? If possible, please share the negotiated speed for connecting the extender's 5G Wi-Fi.
https://www.tp-link.com/support/faq/2265/
In addition, please stand at the extender position and connect to the router's 5G Wi-Fi to measure the speed.

We sincerely want to help you. If you're open to continuing the troubleshooting process, please kindly provide us with the speed results mentioned earlier. This information will be helpful for us to further investigate your issue, and we will ensure that it is forwarded to our senior engineers for additional attention and analysis.

Thank you for your cooperation in this matter.

  @Joseph-TP 

Hi Joseph

I checked the firmware, and it's up to date for the model.

I am definitely connected to the 5GHz

Here are the readings you asked for plus a couple more:

Standing next to and connected to the RE605X extender via wifi: 92.3 Mbps

Standing next to the RE605X extender, but connected to the router via wifi: 66.4 Mbps

Standing next to and connected to the router via wifi: 767 Mbps

On a laptop connected to the Router through the same ethernet cable which runs to the RE605X: 905 Mbps

And finally, standing next to and connected to the RE605X extender via wifi on the 2.5 channel: 73.9 Mbps

Thanks

Gary

  @David-TP 

Hi David

The Router is an Arris NVG578LX

Tried a factory reset, still the same.

The router is definitely not capping the speed. As I stated in my last list of speed checks, I connected a laptop to the cable that runs directly to the RE605X and I got a speed of 900+ Mbps

Thanks

Gary

  @David-TP 

Thanks David

I have received you email. I'll have to pick this up again when I return from holiday in 10 days.

Gary