Let's take an example: 9600 baud. That's 960 bytes per second. So one host is sending data with a speed of max. 960 bytes per second. If the receiving host is able to process all the 960 bytes coming in that one second it's find no matter how fast the processor speed of the sending or receiving host is. The PC cannot send more than these 960 bytes because the baud rate doesn't allow that although theoretically the PC could have done much more.