Since the change [1] which is supposed to "optimize" Special:ActiveUsers for massive wikis (bug report [2]), the page is completely unusable on small wikis.
The number of users in the list is much lower than it should be according to the Special:Statistics page. For example on ArchWiki [3], the list in Special:ActiveUsers shows about tens of users (or less), whereas according to Special:Statistics it should contain about 500 users. Other example of a small wiki with the same behaviour is e.g. [4], so it is likely not a configuration problem.
The users in the ActiveUsers list are not the most recently active users according to Special:RecentChanges.
The content of Special:ActiveUsers depends on how frequently the page is viewed, which is not very often on small wikis.
The second line on the ActiveUsers page says:
"You are viewing a cached version of this page, which can be up to 29 days, 23 hours and 40 minutes old."
On large wikis such as Wikipedia, this value is usually ~1d and the smaller the wiki is, the "older" the served page is. By common sense, this should be inverted as small wikis don't need as much caching.
The `auactiveusers` parameter of the API's `allusers` list [5] is also affected by caching, which means that queries using this parameter give incorrect results. The only way to get correct information is to read `recentchanges` list [6] and manually get the necessary information, which I'd imagine is more expensive query.
The final problem is missing configuration. If the caching of Special:ActiveUsers is not completely reverted, make it at least configurable so that small wikis can disable it to avoid side effects of the caching.
[1]: https://gerrit.wikimedia.org/r/#/c/110299/
[2]: https://phabricator.wikimedia.org/T43078
[3]: https://wiki.archlinux.org/index.php/Special:ActiveUsers
[4]: http://wiki.gentoo.org/wiki/Special:ActiveUsers
[5]: http://www.mediawiki.org/wiki/API:Allusers