See Generic Query Plans in PSQL 9.1. In that page we describe how PostgreSQL 9.1 wanted to use generic query plans for prepared statements, and how these plans were slower that what would be planned and executed for a regular query.
In addition, after upgrading to PostgreSQL 9.4, we've found that adding a compound index on the `triples' columns `subject`, `predicate`, `object`, and `context`, but in a different order than they were specified in an existing index, sped up some remaining slow queries.
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