If your web server is set correctly, through rules in .htaccess that provide Cache-Control and/or Expires headers, jQuery is downloaded once and that’s it. All subsequent accesses on other pages get a 304 Not Modified response and no download happens, so there is no extra added to those pages at all. The browser has it cached locally. Additionally, if the Google-hosted jQuery is specified to be used instead of the one that WP installs, it is more than likely that a visitor’s browser has cached it already by visiting another site prior to visiting yours.
A lot of plugins use jQuery.
If you’re concerned about speed, I recommend the Use Google Libraries plugin: http://wordpress.org/extend/plugins/use-google-libraries/
It switches the native WP JS libraries to pull from Google’s CDN instead, which adds a tremendous speed advantage as well as allows your site to take advantage of caching of those libraries across many sites that use Google’s CDN as well. I’ve seen large speed increases due to this.
I’ve been using google-ajax-libraries-api-plugin. Lean code – 68 lines. Works in mu-plugins/, so no site-wide or per blog activate. Even though it is vintage 2008, it plays with WP3 just fine. Moved to it when Use Google Libraries gave some trouble; can’t remember now what it was–perhaps wasn’t v3 ready at the time.
Just looked at the code of Use Google Libraries again and it has more checks/fallbacks and swaps out more libraries, so will give it another go to compare.
