I don't understand your question well. It takes more time than you expect to Xmit one byte (not the expected baud rate)? OR- The ROMX instruction takes too long and there's a long wait from the time one byte is finished with being transmitted until the next is started to transmit.
ROMX is very slow.
I am not quite sure, but what is the question asked?
Looks like an explanation of why UART data sourced from FLASH compromises
allowable UART datarate vs from RAM, posted as a fun FAQ from a Cypress