You've got to understand what is going on with the hardware and how windows handles interrupts and buffereing to see where the problem is. On a fast enough machine, you may never see an issue. On a slower machine, windows itself and the buffereing in the hardware is the problem.
To give an example, I have an i7, quad-core hyperthreading laptop. With that much power, i should never see stickyness in the mouse or delays between pressing keys on the keyboard and them appearing on screen. The laptop came with a poorly behaved application running in the background, which at times would hog up a lot of resources and make the user interface perform as described above. Now that that program has been removed, I don't see these issues.
That interference with the keyboard an mouse will happen with the serial port as well as it is not one of the higher priority system interrupts. On a display, you will be able to see it... Of course, the spectators might not notice, but you will.
The Open DMX adapter relies on windows for both data and timing. The Lynx Dongle and more expensive DMX adapters have larger hardware buffers and internal processors to isolate the output generation from the limitations of wiindows.
