I also happened to have this old PC with only a slow internal harddisk. It slows down and even pauses for a while if it has to do too much reading and writing from/to the same disk at the same time. Are you thinking what I am thinking? Probably not, unlike you like me happen to have a USB hub and a handful of USB sticks lying around.
In the end, I only installed the game and its expansions on two different USB drives, and relocated My Documents to a third. There is still the occasional momentary pause in live mode, perhaps half a second, but it is pretty rare. So for now, I guess I'll keep it this way.