Yesterday, my girlfriend noticed a severe drop in her fps. From 60 to 20. We're not sure what caused it, but it appeared to have taken place after the 1.0.6 update. When we looked into it further, we noticed the version she was using of Java was Java 64bit Version 7 Update 1. I was using Update 9. She did a complete java reinstall to the latest version and her fps doubled without even putting optifine on.
The moral? Always keep your Java up to date.