I bought and setup a TL-WPA 4220 TKIT AV600 Powerline Kit (TL-WPA4220 x 2 and TL-PA4010 x 1)  yesterday. I set it up right next to the fibre router, and configured the extender next to the TL-PA4010 . My fibre speed is 100Mbps and when testing with a cable direct to the the router, the speed is pretty close to 100Mbps.

Using the tpPLS app, the data rate to the extender is 376Mbps

The WIFI speed test from the TL-WPA4220 extender is ok, and tests around; DOWNLOAD 49.61 Mbps,  UPLOAD 87.25Mbps 

The speed seems to be very consistent and stable, which is good.   

Why is the download speed so much slower? 

I'd appreciate any advice - Thanks