If you're reading a plist to find a match for a file you no longer have a constant algorithm time. You're then running into a search algorithm time which is an O function. The search time becomes dependent on the file you're looking at, which at worst case is O, where O is the number of files that Shapeshifter is theming. Therefore, the more ShapeShifter themes, the worse your maximum possible speed hit is.

What Blacktiger was talking about was something like my example above, where the code has a constant run time. In computer science something with a constant run time is ignored when looking at algorithm effectiveness. Big O notation simply drops anything constant.