Since the change  which is supposed to "optimize" Special:ActiveUsers for massive wikis (bug report ), 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 , 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. , 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  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  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.