Performance Issues with 9.0.1

What could be the reason for the case when fps is 60 if the game is run on max graphics settings from within battlenet launcher and fps is 10 when the game is run on minimal settings from within isboxer?

According to the tests I have done, probably the character set in isboxer has more than 5 slots. Try to create a new set with 5 or less slots. Good luck.

I've got just 2 characters in the slot. When i run just one WoW instance controlled by isboxer, the fps drops to 10 on minimal graphics settings. When a WoW instance is not controlled by isboxer, for example, if I double click WoW.exe, the fps is 60 on maximal graphics settings. This is why we'd like to ask you to take a look at the issue.

Try setting your game profile to DirectX 11 legacy