It seems to me that the "activeusers" parameter in the allusers API-query [http://www.mediawiki.org/wiki/API:Allusers] is quite heavy on the database.

I tried

http://en.wikipedia.org/w/api.php?action=query&list=allusers&augroup=ipblock-exempt&aulimit=100&auactiveusers=1

earlier today which was "Served by mw7 in 16.976 secs". It's probably in an SQL cache now because I can get the result much quicker now.
Seeing that I followed with a request that is not filtered by group, which timeouts:

http://en.wikipedia.org/w/api.php?action=query&list=allusers&aulimit=1&auactiveusers=1

Looking at the code I see that it's implemented as a grouped JOIN of recentchanges and user with an un-indexed WHERE; I'm no MYSQL expert but that doesn't seem like a good idea on enWP.

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