Hello Claude,
I'm not sure that we can decrease the cpu load below the 10 to 14 percent that you observe right now but there are a few things left that we can try to narrow down the the main cause of the load.
If I understood you correct, you already tried to let another master query the slave and have fhem in passive mode. Since this resulted in the same CPU load, the load can not come from creating and sending the requests. In passive Mode the Modbus Module just listens and does not send requests itself.
So the load must come from receiving or parsing. Reading Modbus RTU via serial line might be more cpu intensive than Reading Modbus TCP so just for clarification you could try to configure one raspberry pi with fhem as a Modbus relay that speaks Modbus rtu to your alarm system and Modbus TCP to a second raspberry pi.
Depending on your observation we should know where the main load is cased and maybe find some optimisation there.
Regards,
Stefan