@Richard1313 

If you're experiencing low speed issues when connected to a TP-Link powerline device, there are several potential factors and troubleshooting steps you can take:

1. Check powerline adapters' placement: Ensure that the powerline adapters are plugged directly into a wall outlet and not into power strips, surge protectors, or extension cords.

2. Test different electrical outlets and move them into the same room for comparison.

3. Check powerline rates between the powerline devices via tpPLC utility/App, you could refer to this FAQ to understand the difference between powerline rate and actual internet speed: Explanation about the Powerline Rate and Actual speed of the Powerline Adapters

If the powerline rate is too low, refer to this FAQ to see if it will be better: What can I do if my powerline rate is very low? 

4. Check available firmware updates and ensure the powerline adapter and extender are both using the latest firmware.