@viktor0704 First of all, for improved signal quality on the PLCs, plug your other devices INTO the PLC not besides it. The PLC adapters contain noise filters in the power passthrough, so that your devices and power supplies do not interfere into the wall socket and PLC signal.

Then check your physical connection bandwidth (using tpPLC utility). Given the "PLC1000" class of the adapters, this will be (much) less than 1000 Mbit/s in real life. This is gross bandwidth. Net payload throughput will be about 1/3 of that, as you can see in your current situation. Physical bandwidth is 125, actual throughput is 37.

In a nutshell, 35 is weak and you can improve on that, but don't ever expect anything close to 1000.