Why is dir search quadratic?

[Kenneth Almquist]

The time to search a directory for a single file is roughly proportional
to the size of the directory.  What is quadratic is the time to access
all the files in a directory; if you double the number of files then there
are twice as many files to access and the access time for each file is
doubled, so the time to access all the files is quadrupled.
				Kenneth Almquist