Indexing SQL LIKE Filters

The SQL LIKE operator is a common cause of performance issues because the performance is vastly depending on the search term. But it's not the selectivity of the entire search term that determines the performance—it's the prefix selectivity that matters. In that context, the prefix is the substring before the first wildcard.

http://use-the-index-luke.com/sql/where-clause/searching-for-ranges/like-performance-tuning