If several searches, with no specified date range happen, it can cause the forums to just lock up for around 5 to 10 minutes.
In those instances what happens is SMF times out, but does not kill the SQL thread and then the user starts the search again. Rinse and repeat. Once there are about 5 or 6 of those things running, the user can no longer start any more searches.
Then they come to the board complaining about the search function, once they can get back to the board. Truth is, if everyone used a reasonable date range, or we simply shuttled off the first eight or nine years of posts, it would solve the problem.