hello...
there's no real correlation between speed and your folder structure or amount of files in a folder unless you are on a site with millions of hits a minute and you have thousands and thousands of files...
from a usability standpoint, you wouldn't want to have to make people try to remember something like:
http://foo.com/this/is/a/long/url/directory/string.html
as opposed to:
http://foo.com/shorter/string.html
but that would just be for ease of use to the user...
hope that helps!
sitruc