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Special:Watchlist has a number of hard-coded parameters that could be defined in
DefaultSettings.php, allowing them to be customised in LocalSettings.php so that
site administrators could more easily override them with relevant values.  Some
of the parameters seem to be optimisations to avoid expensive database queries.
 The SpecialWatchlist.php parameters are:

1. the displayed time-limit links using fixed arrays (1h, 2h, 6h, 12h, and 1d,
3d, 7d),

2. the three variables controlling the default time-limit (1000 watched, 12h, 3d),

3. the percentage of articles with talk pages (15%), and

4. the number of articles edited per day (40,000).  This last optimising
parameter is currently only set in REL1_4, which may be a separate issue
affecting performance in 1.5 and later.