Powerline products transmit data through the power line of your house. The powerline rate is mainly decided by the quality of power line environment. When your powerline rate is low, you can do these below to improve the quality.
Please check whether your powerline adapters are separate by power outlet. If yes, please move the adapter on the outlet to the wall.
Please check whether there are electrical equipments which have electromotor inside around the powerline adapters. Such equipments can be air-condition, washing-machine, electric-fans etc. Please move such equipment to other place or move powerline adapters away from such equipment.
What is more, you can buy a power outlet which has surge protector function and plug such equipments to the outlet to avoid interference.
Please check whether there are AC power adapter that used for mobile phone, laptop or something else around the adapter. If the quality of AC power adapter is not good, it may take great noise to the power line circuit. Please move it to other please far away from the powerline adapters.
Please check whether the distance between the adapters is too long. In theory, the biggest distance is 300 meters. However, with the interference and line quality, its hard to reach 300 meters.