The image verification was only in place (as far as I could tell) for when you wanted to search the forum but were not logged in. Unless something changed, I only noticed it when I wasn't logged in.
The purpose of this feature in vBulletin is to prevent non registered users from overwhelming the database by running multiple searches (and in turn shutting or slowing down the board)
That is correct, you need to be logged in to make the search engine easier to operate. The 40 second delay has already been discussed in previous threads and was part of a trade off to make the entire site and especially the serach feature itself function better.