It looks like from my VB6 program I only get one opportunity to send data to the virtual com port once every 4 ms. It's apparently a windows/USB thing and there is nothing I can do to change that. I have rewritten my VB6 program to send larger blocks of data and that has helped.
Now that I am sending larger blocks of data I have another problem. The RX butter on the Arduino is only 128 bytes. Is there a way to increase it? I would like to be able to set it to 256 bytes.