The very fact that your bringing this up means that its obviously something thats important to you (intelligence , i mean), so the option of forgetting about it and carrying on as you are really isn't an option at all.
It seems to me (and i could be wrong here) that you don't see her as equal to you. And if you don't see her as an equal, you can't really respect her in the way she (or anyone) deserves to be respected. Therefore, why don't you just spare her and yourself the misery and heartbreak?
If you enjoy spending time with her, then make an effort to stay friends.
Its better to hurt someone a little bit now that a lot later.
HTH, G Luck! x
p.s. Don't EVER tell her that the reason your breaking up with her is because she's stupid... seriously, just don't.