Hello @stevenbrs,
The ModbusTCP server on localhost:502 is only for communication with the inner hardware of the PLC, not with the extensions.
The extensions are to be connected by RS485. In the OS, you have this HW serial port accessible as /dev/extcomm/0/xxx, depending on the order of the serial port on the PLC (RS485 1.1 is /dev/extcomm/0/0, RS485 2.1 is /dev/extcomm/0/1,...).
With situation like this, you need to convince the Home Assistant to communicate by ModbusRTU protocol via mentioned serial port.
The communication of the ModbusTCP server with the inner HW of the PLC is reasonably fast. The speed problem you are experiencing almost always lies in the software which communicates with the ModbusTCP server. We have seen it across all diferent kinds of applications even with our very own Mervis. Few weeks ago, we have discussed this on another thread: https://forum.unipi.technology/topic/694/unipi-neuron-home-assistant-setup/2
Unfortunately we don't have any direct experience with the Home Assistant, so I am not able to give you any direct advice. Maybe @Martijn-Hemeryck could chip in:)?
Best regards,
Martin