Browser add-on Flickr Gallery Plus adds extended functionality to Flickr's set view, allowing users to click through to see full versions of each shot without having to reload the page. Once installed it will go out and pre-fetch the larger sized version of each shot, making big sets a cinch to speed through. This includes the addition of keyboard shortcuts (something you can't get in Flickr without a Greasemonkey script) which lets you go back and forth between shots using your arrow keys.
In addition to its thumbnail optimizer, the add-on includes a simple slideshow viewer that doesn't attempt to replace Flickr's own gorgeous slideshow tool. Instead, it simply fades together the pre-fetched pictures without leaving the page. You can set how long you want each photo to be displayed, anywhere from one to five seconds.
If you're a heavy Flickr user this extension is definitely worth the download as it will cut your page loads in half. It retains Flickr's slideshow tools and its same navigation; you also have the option to turn it on and off from a setting at the bottom of your browser.