The data rate displayed on the Powerline 1000 (as well as all Powerline 1000 on the market) is the maximum data rate used for the transmission of signals on the link. The useful speed is lower than the maximum speed because the transmission of data always requires a significant use of the band, which reduces the available bandwidth.
The quality of your electrical installation can further reduce the available bandwidth and you will usually get less than 1000 Mbps.
An overly extensive or old electrical installation can significantly increase the data rate loss. Potentially disruptive electrical devices left plugged in unnecessarily can also significantly increase throughput losses. (e.g., telephone charger, power transformer, etc.).
For information, to transmit an HD signal, 5 Mbps is sufficient, to transmit a UHD (4K) signal, 20 Mbps is sufficient.