Hello @Algra 
As range extender needs to receive data from the router and then forward it to the connected clients and speed will decrease during the transmission, if range extender speed is not satisfactory, it is very important to check what speed it received from router at the same location,can you help confirm some details here so we could check if its a normal speed

- at RE's location, what is router's 2.4Ghz speed and the Wi-Fi link speed on computer (TL-WA855RE only supports 2.4Ghz, make sure you are testing 2.4G at that time)

- what is the speed of rannge extender and the Wi-Fi- Link speed on computer

How to improve the speed of TP-Link wireless adapters?
TL-WA855RE supports 300Mbps on Wi-Fi Link speed,  it is recommended to find a laptop which wireless adapter supports 300Mbps and see if speed test results could be better. 

By the way, may I know the model of your Mi range extender? 

Hello @Algra Thank you for your detailed reply. As range extender works in half- duplex (receive and send data at different times) and the speed loss in the wireless transmission, the speed can be less than 50% of the speed that extender receives, for a single band extender. 

At second floor hallway, how much speed you could get from router at extender's location?  And what is the Wi-Fi Link speed on your client at that time? 

If the Link speed  is higher, the real speed test could be higher * also limited by the speed extender receives. How to check the associated/link speed on a wireless client

Please also confirm 855RE firmware is up to date: https://www.tp-link.com/support/download/tl-wa855re/#Firmware

You could also put Mi extender at the second floor and see how much speed it could get. Can I have photo of the speedtest results on router (at RE's location) ,extender and the link speed.