Currently the wiki has 2,000,000 outdated entities. This not just inflates the database but also reduced the performance. Since the wiki is on SMW 2.5.x the following daily script exection will be helpful:

php /path/to/SMW/maintenance/rebuildData.php --skip-properties -s 1 -e 1 --with-maintenance-log

Before creating the cron job I recommend to clear out the existing outdated entities manually once.

See disposeOutdatedEntities.php for further information. Note that the preferred way depends on the version of SMW used.