The specs for your system look like it should run Kodu just fine.
First of all, which version of Kodu are you running? Is it the one from the Windows Store or the desktop version? If it's the oine from the Windows Store I would uninstall it and install the desktop version instead. You can get it here:
Next, turn on the frame rate display. You can do this by looking in the Options Menu off of the Main Menu. It will be the 4th option in the list. This will display the current fps values in the lower left hand corner of the screen.
Now run Kodu. What is the framerate on the main menu?
Go into the Load Worlds menu and search for the built in world "Roadkill". Run the level and check the frame rate.
Let me know what fps values you are seeing and we'll keep trying to figure out why the performance you have is so poor.