It's hard to idenitfy problems without seeing some code.  Maybe you could provide a link?

Still, a common and maybe relevant issue that causes slowdown is repeated loading of files within draw() - something that should be avoided wherever possible.  Also, remember that the applet is embedded in the browser and that's going to hog some resources and potentially slow things down a little; but my guess is that it's an issue with file operations...

Multithreading can be an issue as well, I don't know if browsers can limit it in the applets they host.
If you want to publish a large sketch, you can do it in one of the numerous pastebin (google the term) clones on the Net.

Chrome (on the Mac at least) does have some pretty hefty problems with rendering Processing.

Hello everyone,