According to some websites, the max baud rate for your PLC is 19200 (which these days is considered slow).
If you do read the topic I mentioned in one of my previous posts then you will see what AabeckControls did in his project to improve the performance (the same changes made to the driver might not be possible in your case unless Archie comes with some solution but shuffling things on your forms might help).
Maybe, if your project had only 3 forms then it could possibly be working OK.