I have 12 ~200 MB files. Copying one of them to another drive takes 20 seconds. When I try to copy all of them at once, it first says it's going to take 5 minutes, then it keeps slowing down into the hours and days.

I'm copying the files through Thunar in XFCE, but I remember having similar problems using the command line. Why does this occur and what can I do to remedy it, other than just copying the files one by one?