Why is my Powerline adapter connection under 100 Mbps?

I have a fiber line with a speed of 1 Gbps. When I plug a PC directly into the router I get around 900-950 Mbps.

I’m trying to get the internet to other rooms in my house using Powerlines adapters. I’ve tried 2 PowerLines: 2 x Netgear 500Mbps and 2 x Devolo 1200Mbps.

In both cases (with Netgear or with Devolo) the internet speed that I get on my PC in other rooms is under 100 mbps (85-90Mbps).

The distance between powerline adapters is low 5-6 m.

Why? What can I do?