Siemens PCS7 and ION7300 Meter & Protection Relay

We are experiencing a slow communication between Siemens PCS7 DCS using CP341 Modbus Master and ION7300 Meter & Protection Relay as Slave. We have 18 interconnected units of ION7300 with RS485-serial connection (using looped twisted-shielded-pair cable) to the Siemens CP341 Modbus interface with ET200M and Fiber optic Profibus Network in a remote cabinet going to our PCS7 DCS. We are getting at least 12 parameters in each of these IONs. What seems to be the problem or would you recommend to speed-up the data transmission?