Why does the user search only activates after at least 3 letters have been typed?
Searching for tags does not seem to have this mechanism.
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Sign up to join this communityStack Overflow has 11 million users. Auto-searching after typing just a single character would generate far too many results to be useful (there are probably at least a million usernames that begin with "S", for example), so it waits until you've typed at least three characters before doing anything.
Not only are there far fewer tags than users (60,000 tags vs. 11 million users), but there's also an easy way to guess which tag a user's looking for: list them in descending order of use. For example, if you've typed a "J" in the search box, it's likely that you'll want either javascript or java.