@5665785765 

To ensure range extender get expected speed, you could check some tips in guides below: 

• Confirm range extender is placed properly:  How to place your wireless Range Extender for optimal reception and performance
• Refer to guide here to  confirm whether my range extender is successfully configured and in the best location

We expect range extender to extend the Wi-Fi range in the Wi-Fi dead spot more than to improve the speed, this relates to how range extender works.  Range extender receives data from the router wirelessly and then forward it to the Wi-Fi network clients, the speed will decrease during the transmission. 

As Ethernet connection is usually faster, to boost the speed, you may also consider the Ethernet connection and wire range extender to router via Ethernet cable :

Configuration Guide How to connect TP-Link Wi-fi Extender to Router with Ethernet Cable (Access Point Mode）