I knew I was taking a gamble using a Java applet for SearchMash, since Sun seems to have abandoned it as a client-side technology. Mostly it's paid off, I've been able to develop a pretty rich web app that runs entirely on the client, and with no install step. There's a big problem with using a Java applet though, it takes forever to load, often adding 30 seconds to the load time of the page.
This is a well documented problem, but it seems like a tough one to solve. On OS X with Safari it's not as bad, since it seems like the Java environment is pre-loaded, but it still takes 10 seconds or so the first time you visit the page. On OS X Firefox it's a bit worse, and on Windows both Firefox and IE can take a very long time.
My only ideas on tackling this are to do something interesting during the loading process (a game of pong anyone?), or switch to a different technology entirely.
I've been considering two different approaches, either using a Firefox extension, or writing a standalone app. The Firefox extension would require an install step, something I've tried to avoid, but would be an integrated part of browsing after that. A standalone app would be more work to write, require installation, and have the barrier of not being part of the browser, but it would allow me to do a lot of the more advanced things I've wanted, like animated thumbnail browsing of pages.