Edit: this user was running in GWT hosted mode, which is 10x or more slower than compiled mode. So he was not experiencing an actual performance issue.
We see people complain that their own (flawed) deployments are slow but not usually about the Showcase itself. Most likely, you ve got your browser configured for development or have tools installed that are slowing things down.
First, see if you have set your browser to non-default cache settings (such as "check every time"). This could cause images to be repeatedly re-requested. You can see this if you open the "Net" panel in Firebug, or with IE, use a tool like Fiddler.
However, about Firebug, Firebug and similar development tools greatly slows down the performance of the browser and leaks a great deal of memory. To see the Showcase how a normal end user would see it, disable Firebug and restart your browser in case it has been leaking memory.