Typically, you can tell the order between two strings early, after you read just the starting few characters. So for comparison you would usually read maybe 10

Typically, you can tell the order between two strings early, after you read just the starting few characters. So for comparison you would usually read maybe 10 bytes per file on average, or 3 x 1000 x 10 bytes = 30KB in total. Or maybe 30MB if the strings have long common prefixes. Of course, in a highly specialized distribution, all the strings could be identical except for the last few characters, and then sorting would be horrible. Ultimately you have to have some idea about the distribution of your data to say anything about average complexity.