Hello  @MatCev 

Thank you for your feedback.  As Range extender receives data from the router and then forward it to the connected clients and speed will decrease during the transmission,  the speed of router at RE's location is quite important. 

• For dual-band RE, when perform a speed test, try test RE's 5Ghz Wi-Fi band which usually has faster speed than 2.4Ghz wifi
• you could refer to the thread below to troubleshooting

 What should I do if Internet speed becomes slow via Range Extender?

Can you help  provide some  information below to check if the speed is reasonable:

1. Speed test result from router's  at the RE's location, this is to check how much speed range extender could receive speed from router ; Wi-Fi  link speed  (How to check the associated/link speed on a wireless client)

2. Speedtest result of RE's 5Ghz Wi-Fi  & Wi-Fi  link speed 

Thank you