The most likely reason for performance degradation is that the communication port is not running a buffered UART (Universal Asynchronous Receive/Transmit serial port chip).
Go into the Advanced settings for the serial port and verify the check mark is checked to use a buffered UART. Make sure the Receive Buffer setting is not set to the right-most setting.
If that does not help and you suspect that there is a possibility that the PC may truly not have the buffered UART, IBM® recommends that you slow the connection down between the PC and server. Depending on the operating system, you may have to change the registry, the DUN object, or the phone book entry, or all three.
The problem when the UART is not buffered is that the high speed inputs data to the UART faster than it can handle, which causes a missed packet of data resulting in a 30-second retry. This can occur randomly, but will be persistent. The slower speed reduces the exposure to an overrun of data and therefore no more 30-second retries.