For each applet MozillPlugin creates a Timer which polls windowLocation 60 times per second. When a number of applets (10-20) are loaded on a page the CPU load of the browser + plugin-process might be up to 50-70%.
May be it makes sense to reduce the polling frequency depending on the number of applets, i.e. share those 60 times/sec for all applets to reduce the total load.

The sample app could be run from:
http://nemo.sg.oracle.com:8080/manyapplet/demo.html
http://anashaty.ru.oracle.com:8080/many (20 applets)
http://anashaty.ru.oracle.com:8080/many/5.html (5 applets)
http://anashaty.ru.oracle.com:8080/many/10.html (10 applets)
http://anashaty.ru.oracle.com:8080/many/15.html (15 applets)