@Th3ry Hi,
Please put powerline units into the same room, into Wall sockets, and check speed again. Does powerline unit work fine when they are in the same room?
Please make sure firmware on your powerline unit is up to date:
https://www.tp-link.com/support/faq/1091/
To see if this issue is caused by poor power line environment, please download tpPLC Utlity and share a photo of the powerline rate between 2 powerline units : https://www.tp-link.com/support/download/tl-wpa4220/
You could access Web page through the Global icon on tpPLC Utility: https://www.tp-link.com/us/support/faq/1417/