Manual:$wgMiserMode

Details
If true, disable database-intensive features, so that they can be managed/controlled separately if desired. This includes reading special pages marked as expensive from the cache instead of regenerating them every time they are requested.

With  set to , this means that the special pages managed by this flag will only be updated when explicitly made to update. For example, setting up a cron-job to call  or using Extension:RefreshSpecial.

The other disabled features are...

In MediaWiki 1.23+:
 * Disables custom formatting of change sizes in Special:RecentChanges via MediaWiki:Rc-change-size
 * Disables selecting all pages that start with x box on Special:Log (and  option of   API module)
 * Totally disables Special:MimeSearch, as well as  and   option in the   and   API module
 * Disables  option of the RSS feed for Special:Contributions
 * In the  and   API module, use reduced sorting by namespace mode (returns only a few results when   or   option is in use).
 * Similarly disables the filter by namespace box on Special:LinkSearch
 * Disables showing list of most popular pages on Special:Statistics (but will still show other page view statistics if counters are on)
 * Doesn't update number of active users quite on Special:Statistics as often (?)
 * Disables the search for images with x somewhere in their name box on Special:NewImages and Special:ListFiles
 * Do not show how many previous versions of an image were uploaded on Special:ListFiles
 * When running rebuildrecentchanges.php maintenance script, will not re-auto-patrol edits by users with auto-patrol flag.
 * When running updateCollations.php maintenance script, will not tell you how many rows in total there are to update.
 * Disables transclusion count on info action as of 109710.

For reference, this wiki has $wgMiserMode on. You can tell if a wiki has miser mode enabled via the  API module.

update.php
This configuration option disables update.php in 1.18.0 (and only 1.18.0. Subsequent releases will not use this option for that purpose), requiring one to use  to run that script. Very very very large wikis (like Wikipedia) may need to have more careful upgrade process than the maintenance script does, but most sites would not have to worry about this.