And that one is pretty big. It's the jQuery file, and it's huge by Web standards. Compressing it would save 80 KB per request. That may mean up to a quarter of a second of waiting, or even more in the case of mobile visitors to your site.
Well, we can't wait until the Plone developers fix it at their characteristic glacial pace, so we'll hack around it with Varnish. It's pretty easy to do with Varnish 3's automatic gzip compression support.
The following VCL does the trick: