If your using javascript in your rails application, it's a good bet you have this line in your application layout file.
<%= javascript_include_tag :defaults %>
Problem is it pulls in a whole bunch of libraries, many of which you may not be using. So the best thing to do is specify only the scriptaculous libraries you need, for example, in nearly all my projects, effects.js is all I use.
<%= javascript_include_tag "prototype.js", "scriptaculous.js?load=effects", "application.js", :cache => "all" %>
This reduces my javascript hit by 63%, and results in a noticeable speed increase when first visiting my site.