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"In RT for ages ACL are checked after search what in some situtations
result in empty search pages and wrong count of tickets.  Set
$UseSQLForACLChecks to 1 to use SQL and get rid of these problems. 
However, this option is beta. In some cases it result in performance
improvements, but some setups can not handle it."

Depending on how many users/groups you have and how your database is
configured you may see a performance hit, maybe give it a try on a test
instance and see how much of an impact there is.