Hello @brvus76
I don't really know why the communication might be slow. I can only guess that UIFlow firmware maybe has some safeguards in place to prevent serial from flooding the line.
I guess that your software on the PC manipulates the control lines (CTS, etc.) on the serial port in a way which triggers the auto download / reset mechanism in M5Core2. Unless there is some setting in your software to configure that behavior I don't think there is much you can do.
Thanks
Felix