Clearly, the limiting factor in your application is the serial port throughput (which includes both the data transfer times and the turn-around times of the devices you're talking to). Still if you are plotting the data to say a stripchart, the speed shouldn't be a problem.
Can you describe the interface a little more? How exactly does the serial interface to the instruments work? What is the format of the data coming back? What are you needing to do with the data after you've captured it?
Mike...