Does a wireless extender slow down wifi speed?

My gut feeling it shouldn't but my real life experience is it does and I can't work out why.
Here's my situation. I recently changed ISP to a fttp service and was supplied with a Technicolor ac router. On my laptop and mobile I'm seeing speeds of around 375Mbps down from a different room to the router..Thinking I'd get better wifi coverage throughout the house I got a TP Link ax extender but to my dismay when I set it up my wifi speeds drop to around 250Mbps instead of the 375ish I had before, devices speed measured in the same position.The devices are using 5G channel not 2.4.
If I turn off the extender the speeds rise back to the previous level.
Anyone have any idea on what can be causing this?

In general, if the ap doesn't use its own backhaul channel or wired backhaul it will reduce wifi speed sometimes by up to 50%.

I assume you want better wifi coverage in your property?

better coverage yes.
I'm not using the device as a AP just a wireless extender.

Post deleted by ian72

You will have a much better experience purchasing a dedicated wireless access point and running a cable to it. The UniFi devices are very good, and can be managed via an app on your smartphone.

Wireless extenders are snake oil IMO, they either flat out don't work, massively cut your connection speed, introduce lag and general instability.

We have a TP powerline extender for the far end of our house, and it cuts our 70MB connection by more than half. The wifi connection to Sony TV is flaky so I use a short ethernet cable and it works fine for the past four years, unlike the Devolo gear. Soi my answer is yes, extenders do slow things down.

The Powerline committee has shut down. Powerline wasn't able to keep up with the speeds that WiFi could reach easily.

Most "extenders" work by receiving a WiFi signal, and then transmitting it on a different channel/frequency. If you pick up a 20 MHz wide 5 GHz signal and transmit on 20 MHz wide 2.4 GHz this MAY be close in performance. All other combinations will result in slower speeds.

A proper mesh system using hub/spoke topology over wired backhaul is best, but mesh systems that use WiFi as backhaul CAN work better than simple "extenders" and much easier to configure.

I use TP-Link EAP PoE AP's and they are very good. Easy to setup and manage via a browser or app on a phone. They do not interfere in any way with the Wifi speed.

I would second using a dedicated Unifi AP. They do also support wireless meshing but try to wire them over PoE for best performance.
There are some older models which are still perfectly usable (I'm using AP-pro) albeit maybe without the absolute fastest speeds but still able to work at 600mbit and are availble 2nd hand from £30 with a PoE injector. They are so simple to setup using the Unifi software too.

In reply to a post by jchamier:

The Powerline committee has shut down. Powerline wasn't able to keep up with the speeds that WiFi could reach easily.

Ahh i didn't realise that had been shut down. I have noticed a decline in new products.

In reply to a post by jchamier:

Most "extenders" work by receiving a WiFi signal, and then transmitting it on a different channel/frequency. If you pick up a 20 MHz wide 5 GHz signal and transmit on 20 MHz wide 2.4 GHz this MAY be close in performance. All other combinations will result in slower speeds.

Just gonna mention if the extender picks a congested channel to retransmit, you may find slow downs. Also, if the wireless extender only recieves say 5mbit/5mbit, then you will only have that speed regardless of the connection to the extender. I know you that you know this, just saying for the masses who may not.

In reply to a post by jchamier:

A proper mesh system using hub/spoke topology over wired backhaul is best, but mesh systems that use WiFi as backhaul CAN work better than simple "extenders" and much easier to configure.

even with a dedicated radio for wireless backhaul, you are facing the same issues that your wireless router faced. A combination of wired and wireless backhual would better but may only provide good thruput in ideal conditions - ie good wifi signal.