@Sylven Good day, 

Thank you for your detailed post ~

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, we use it more to increase the stability in Wi-Fi dead spots rather than boost Wi-Fi Speed.

But in your case the range extender speed should be a little higher.  May I know did you test router's 2.4Ghz Wi-Fi speed or 5Ghz Wi-Fi speed?  As TL-WA850RE only supports 2.4Ghz,  please test the2.4Ghz Wi-Fi speed from Router at  the Same Location of RE .

From the photo,  WiFi link speed between RE and laptop is 144Mbps, while TL-WA850RE supports 300Mbps on Wi-Fi Link speed, 

So you could find a laptop which wireless adapter supports 300Mbps and see if speed test results could be better. 

@Sylven Hello, 

Please put the same laptop close to range extender and see if you could get 300Mbps, if not ,may I know the model number of the wireless adapter or model of your computer? 

May I know the model number of your Wi-Fi router as well, thank you. 

@Sylven Hello, 

Thanks a lot for your update. 

Range extender's speed looks more reasonable if router's 2.4Ghz is around 70Mbps considering more wireless interference on 2.4Ghz.

You could consider a dual-band Wi-Fi range extender if you need stability as well as relative faster speed (compared with single band RE) at that Wi-Fi dead spot. 

Yes 5Ghz band can be  multiple times faster than 2.4Ghz .5Ghz Link speed is normally 433/866 Mbps while 2.4G Wi-Fi link speed is normally150/300/450 Mbps, but 2.4Ghz has a bigger Wi-Fi coverage and some Wi-Fi clients only support 2.4Ghz.