No, it is not. This is a claim which is testable and measurable, and for reasonable assumptions, you can predictably, measurably, and in a reproducible fashion improve page load times by implementing the YSlow recommendations.

If you have the following line of code in application.rhtml:

javascript_tag "prototype", "scriptaculous", "shopping-cart", "color-picker", "mysite-functions"

and you add these 16 characters

, :cache => true

then every page on your site now loads faster.

P.S. Explanation for non-Rails users: the above code causes the five specified Javascript files to be concatenated into "all.js" the first time any page on the Rails site is rendered, and all pages load all.js rather than loading the five Javascript files. This cuts the overhead of four HTTP request/response cycles from the page. It gives an obvious, measurable impact to page load times, and plays well with another braindead easy optimization "set your HTTP server to gzip outgoing textual content (including Javascript)".