How do I add a checkbox to the performance page and have it repopulate?
to have it "repopulate" after save provide a #default_value key that retrieves it on page load.
to actually save it append (add another) custom form submit handler to the form, after the original one gets fired. Something like $form['#submit'][] = 'mymodule_mysubmit_handler'; You will have to check where the submit handler is set already in the form, or the element you're trying to hook into (such as $form['caching'] ...