You have to asscertain that your program is not writing out of the RS232 port in any command. Any function such as PRINT, OUTCOMM, READMODBUS, WRITEMODUS, NETCMD$ etc will intrrupt the communication between the Maple and the PLC.
The best way to test if there is anything wrong with the PLC program is to put in a simple program or halt the PLC execution. If the Maple still communicate with the PLC very slowly, then it could be a communication issue between the Maple and the PLC. You will need to check if the handshake lines in the Maple are properly looped back etc.