If $8 Raspberry Pi Zero and Adafruit Trinkets can inject the payload instantly I would imagine any PC after Windows 98 should be able to do it too, so I doubt the speed of the actual computer has anything to do with it. Also the fact that it literally sends a few lines of code makes it even more unlikely, my Android phone does it in a split second...
There's something wrong with a cable or driver. I'm willing to bet it's the cable setup.