By default there is a search delay/limit for members and guests. For example: 30 seconds for guests and 5 seconds for members.
When a member or guest viewing "active topics", "unanswered posts, etc and the member/guest is refreshing that page (F5 or hard refresh with CTRL+F5) there is no problem. This is because "active topics", "unanswered posts" etc. are not using the search index database table?
But when a user/guest used the search function (searchbox) they have to wait the given seconds (between searches in ACP) before they can use "active topics", "unanswered posts", etc again.