Yes, I've been meaning to look into that. But I still don't understand the issue. It complains about the sub-account name and when I check they are all correct.
WoW saves the account names in Config.WTF, and you have different accounts saving over the same Config.WTF because of the different Characters all on the same files.

Yes. Are the Config.WTF files created by ISBoxer? Should I edit each copied character to use its own Config.WTF?

ISBoxer creates the Character-specific Config.WTF files, by copying the master Config.WTF file. Whatever settings are in use at the time you make your new Character will be used for that Character. So it can happen where your old characters don't have Antialiasing or something configured, but then you turned it on later or it became a default setting and then the new characters have it in their Config.WTF file ... dropping the FPS.

Yes, every character should be using its own Config.WTF. Or at LEAST each ACCOUNT should have its own Config.WTF.

I tried that and still have the same result.
If the problem isn't WoW video/audio settings (as would be confirmed by comparing/copying the Config.WTF or the in-game options panels), and isn't ISBoxer settings (confirmed by looking at them) unless you forgot to Export to Inner Space, then at that point I might disable some WoW Addons. Then search for demons, and when you're done with that go back to http://isboxer.com/wiki/HOWTO:Tweak_your_framerate and use the tool in the "Identifying framerate bottlenecks" section to capture a report so we can see if that indicates it is GPU, CPU or RAM related...