When the master send a command to slave, and if the slave doesn't respond before time out, or if the slave send back a bad response (wrong LRC), then the master will re-transmit the command up to 2 additonal times. What you think as "delayed" response is probably a response on a resend.
1) How many devices are on the RS485 bus?
2) Are you using only Modbus ASCII protocol or are you switching between protocols?
3) How long is the distance between the PLCs?
4) For longer distance, try to use biasing resistors to put the RS485 in a known state and you may need terminating resistors.