See Bugzilla:
http://bugzilla.mozilla.org/show_bug.cgi?id=129680

Basically, when you have a lot of applet on the page the Plugin slows way down (upwards of 5 minutes to load the page) and sometimes hangs. This only seems to happen with Netscape 6 (not sure about 4) and on Windows. The UNIX plugin does not show the same slowdown (aroung 10 seconds to load the page). The windows plugin with IE works fine also.

Take a look at:

http://j2se.east/skatz/bullet/huebullet.html
and
http://j2se.east/scratch/bullet/huebullet.html

The former loads all the applest from the same codebase. The latter uses a different code base for each applet. (No difference in performance though).