On grade user report we initialise iterator 3 times and its the same iterator:

1. First we do it here while calling get_gradable_users - https://github.com/moodle/moodle/blob/main/grade/report/user/index.php#L120
2. Then we do it here separately - https://github.com/moodle/moodle/blob/main/grade/report/user/index.php#L139
3. And the we initialise it for a search here - https://github.com/moodle/moodle/blob/main/grade/classes/external/get_enrolled_users_for_selector.php#L94

So we run same queries for no reason. We should cache it and reuse instead.

I think it happens on other reports too