@Ronsensei Hello, 

The speed that RE gets directly from router is quite important, as this is a single band RE,   please test what is 2.4Ghz Wi-Fi speed from Router at  the Same Location of RE ? 
Please check the actual speed and Wi-Fi link speed when your device is connected to Range extender & main router separately and share the results here: 
https://www.tp-link.com/support/faq/709/

If possible,  please get a  laptop which can wire to Extender and test RE's  Ethernet speed?
Please also try to modify the wireless channel of the home router, such as channel 1/6/11 for 2.4ghz and see if speed gets better or not. 

Kindly note that range extender get speed from router first then forward to your PC or clients, this communication is one- way only at a time,  RE's speed could decrease around 50% during this transmission.